We have been working on and using custom Qt theming in Fedora Workstation for many years now. By custom Qt theming, I’m talking about the QGnomePlatform and Adwaita-qt projects. If you haven’t heard of them, you can read my recent blog post explaining what they are. While these projects are in some ways better than what Qt upstream has to offer, there were also drawbacks/issues and that’s why I decided to make a final decision and discontinue both projects. The issues are explained in the aforementioned blog post, but one of the main drawbacks is that we are in this development alone and not working directly in the upstream makes it less attractive for other contributors. It’s also not used by default anywhere other than Fedora, so it’s not properly tested by other developers working on Qt applications using different distributions. These reasons led me to submit a Fedora 39 feature to remove our custom Qt theming in Fedora Workstation in favor of Qt’s defaults. The only problem is that if we just go with Qt’s default, we would go backwards a bit. This is because upstream Qt does not provide any decent client-side window decorations (problem #1), and the QGtkTheme in Qt5 (QGnomePlatform equivalent) is a bit behind its Qt6 version with many improvements and integration goodies (problem #2) recently made by Axel Spoerl of the Qt Group, whom I met during this year’s KDE Akademy.
Solution to problem #1
QGnomePlatform used to be our solution to this problem, as QGnomePlatform implemented it’s own version of the QWaylandAbstractDecoration plugin. This was a GTK 3-like decoration plugin that used Adwaita-qt for button rendering and QGnomePlatform bits (e.g. GSettings configuration) to get the titlebar layout. Since we are going to remove QGnomePlatform, we needed an alternative. So I started working on the QAdwaitaDecorations project. This is supposed to be a an intermediate step as I would like to have a proper GNOME/Gtk decorations directly in Qt upstream, but since I was in a hurry to get everything done in time for Fedora 39, we have this for now. QAdwaitaDecorations plugin is based on the decorations we have in QGnomePlatform, but there is no dependency on GTK or Glib (e.g. GSettings) or on Adwaita-qt. We use xdg-desktop-portal to get the titlebar layout and do our own drawing instead. This decoration plugin should also have now a GTK 4-like style so the buttons and colors of the decorations are different.
Below is a screenshot of Wireshark (Qt6) using QAdwaitaDecorations plugin + QGtkTheme + Fusion:
Solution to problem #2
Since Qt5 is no longer actively developed, the only possible solution is to backport all QGtkTheme improvements from Qt6, so I did that + modified some of those changes to avoid breaking binary compatibility. This results in about ~15 related backports to Qt5 so far, and it seems to work pretty well. I also made sure that Fedora 38 and older will still use QGnomePlatform by default, so we don’t change the behavior for existing users. Also, a small change to our QtWayland package was needed to make it use the new decoration plugin by default.
As mentioned, I would really like to have everything directly in Qt upstream (talking about QAdwaitaDecorations). That way we get other contributions and thus fixes/improvements for free and a lot more users. Another thing is that QGnomePlatform supports things that are not yet supported/implemented in QGtkTheme, like support for xdg-desktop-portal instead of just relying on GSettings. Not to mention that GTK 4 has been around for a while, and both QGnomePlatform and QGtkTheme are still GTK 3 based. I will definitely try to make some of these things happen for Fedora 40, but knowing myself, it’s better not to make any promises, as things usually don’t go according to plan.
I decided to write this post to explain everything I can about these two projects. There have been discussions and people demanding these projects should not be used by default in Fedora. As part of this, some issues were raised and it might not be clear which component might be responsible for what. I ended up constantly defending these projects in many discussions and ended up being exhausted by doing so over and over, so take this as an explainer to shed some light.
Brief introduction to QGnomePlatform and Adwaita-qt
To give you some context before I go into details, you can think about Adwaita-qt as the UI representation and QGnomePlatform as the integration between GNOME and Qt. QGnomePlatform applies your GNOME configuration and behavior to Qt apps, together with some integration bits, like dialogs or client-side decorations. Adwaita-qt is responsible for the style of the app itself, including the style of all visible parts (widgets/buttons).
What is QGnomePlatform?
QGnomePlatform is a Qt Platform Theme (part of Qt Platform Abstraction API), where such a plugin is responsible for the app integration into the desktop environment. It is designed to provide integration between Qt apps and the GNOME platform. To explain in an example. Without any platform integration, Qt apps running on GNOME would use default styling and configuration so your fonts, icon theme, dialogs would not fit into the desktop. Also in the case of QGnomePlatform you would not have GNOME-like client-side decorations.
What QGnomePlatform provides?
QGnomePlatform provides the following integration for Qt apps running in GNOME:
Font configuration *
Icon theme *
Cursor size and cursor theme *
Static hints (like double-click time, long-press time etc.) *
File dialog (both using GTK3 * and native dialog using xdg-desktop-portal)
Font dialog using GTK3 *
Color dialog using GTK3 *
Support for Settings portal from xdg-desktop-portal settings
Unlike the built-in GTK3 theme that can get everything only from GSettings
Brings support for light/dark theme switching introduced in GNOME 42
Use Adwaita-qt theme by default (elephant in the room) and Adwaita color palette
Also provides support for additional themes, like Kvantum
Support for Cinnamon desktop
* these can also be provided by built-in GTK3 platform theme in Qt itself (just for comparison what QGnomePlatform does extra)
Issues QGnomePlatform gets wrongly blamed for
As stated above, QGnomePlatform provides an implementation of CSD. It’s actually the only Qt CSD implementation I’m aware of, excluding the reference implementation provided by QtWayland, named Bradient. Below is a screenshot comparing QGnomePlatform (left) and Bradient (right).
I saw many times people complaining about missing shadows support and resizing issues. The truth is that officially there was no proper shadows support in the Qt API until I introduced it with Qt 6.2. That’s the reason we don’t have it for Qt5, unless you are a Fedora user, where this support has been backported and enabled in QGnomePlatform build. I also fixed all kinds of CSD related issues in QtWayland.ven though Qt has proper support for shadows in Qt6 now, the reference implementation doesn’t use them.
This has nothing to do with Qt platform theme or CSD implementation, because it was actually a bug in QtWayland. Unfortunately this fix is only in Qt 6 and cannot be backported officially to Qt 5 as it would break KDE Plasma. I managed to at least patch QtWayland in Fedora and Flatpak KDE runtime, where I modified this patch to not affect KDE Plasma at all.
What can QGnomePlatform be blamed for?
Forcing Adwaita-qt color palette
QGnomePlatform sets Adwaita-qt color palette to each Qt app so applications can use QPalette API to get access to colors used in the style itself. This can be for example useful when an app creates custom widgets that would not get styled by the QStyle itself. This creates a problem for KDE applications using the KColorScheme API.
Examples of this issue:
The reason is that KColorScheme and QPalette are out of sync and there might be color roles that are in KColorScheme, but not in QPalette. If an app requests a color from KColorScheme that’s not in QPalette, KColorScheme will default to Breeze style and a color that’s not going to fit the Adwaita-qt style will be provided, causing the app to mix light and dark colors.
I think this is the most visible and user-facing issue we currently have and get blamed for so I would like to fix this as soon as possible.
Adwaita-qt is a Qt style for widgets. Qt style is again part of Qt Platform Abstraction. You can think of it as a theme for your application. It’s what changes the visualization of your widgets like buttons, checkboxes etc. and it’s the only thing that changes the appearance of the application itself. For comparison, the screenshot below is Adwaita-qt (light) and the second one is the Qt’s default Fusion style used on Linux used without any QGnomePlatform influence so basically what you would get by default.
I can also add that Adwaita-qt supports HighContrast variants, which are useful for visually impaired people.
What issues can Adwaita-qt be blamed for?
I already mentioned the color mismatch issue which is not really Adwaita-qt’s fault. There are of course issues in Adwaita-qt itself and it’s far from being perfect. The whole style needs a complete overhaul, because the last one was done in 2019 and the Adwaita theme changed a lot recently with GTK4. Another issue is that while the majority of common widgets are styled just fine, there are still some widgets that are rarely used and might have issues with this style.
Another issue is that apps that customize standard widgets (e.g. through CSS), might get into trouble. Below is a screenshot of Wireshark (pure Qt app) compared to Breeze in KDE.
Another example can be seen in the Black Chocobo app, which uses some customization:
Should QGnomePlatform get removed/replaced?
Definitely not. It has many benefits and extras compared to Qt’s default platform and most importantly gives you CSD support. Once I fix the color mismatch issue, there shouldn’t be anything users should complain about. Obviously, this would not be an issue when Breeze is used instead of Adwaita-qt, but still an issue when Fusion is used so it would still need to be addressed. You can see a screenshot below showing Fusion style used in combination with GNOME set to dark theme:
Should Adwaita-qt get removed/replaced?
Maybe. It depends on the alternatives. Obviously using Breeze would get rid of all the widget issues one might experience with Adwaita-qt, however, it is problematic to ship it by default due to bringing dependencies on KDE Frameworks and Plasma breeze style. With the default Fusion style you will also get many widget issues fixed, but you still need to set the color palette through QGnomePlatform in case you want Fusion to be “dark” and fit into the desktop, otherwise you will always end up using the default “light” variant no matter what configuration you set in GNOME.
I hope that this post is useful for those observing issues with Qt apps under GNOME, and will help them to understand which component is responsible for what, as well as the issues involved. In case you are interested and would like to either contribute a patch or report an issue, here are links to QGnomePlatform and Adwaita-qt repositories.
While we have pretty good support for screen sharing on Wayland in WebRTC, which is included in browsers like Chromium or Firefox, it is still not enabled by default in Chromium and it is kept behind a flag. Not only you have to remember to always enable it for new configurations, but for many users it is not even something they are aware of. This has been my main focus recently and I would like to share with you steps that has been done and what are the plans for the future.
What are the changes to expect in Chromium soon?
Last year I landed proper DMA-BUF support in WebRTC, which made things way faster. It was working, but it was not perfect and there were some corner cases where it might not be working at all. Here are changes I made recently:
Advertise DMA-BUF support when it is really supported. Older versions of PipeWire don’t handle the new way of DMA-BUF negotiation and therefore it shouldn’t be used in such cases. Also using DMA-BUF modifiers requires some recent versions of PipeWire on both sides.
Implemented stream renegotiation. In situation when we fail to import a DMA-BUF with given modifier, we will drop this modifier and try to renegotiate stream parameters and go with a different modifier or fallback to shared memory buffers in case we fail completely.
Make sure to import DMA-BUF with correct render node. In case of multi-gpu setups, we always picked the first render node to import DMA-BUFs, but it can happen that they were actually produced by a different render node and for that reason we might fail to import them. We now try to get default EGLDisplay, which should be the same one used by the wayland compositor and we should be using same render node.
Until now we had mouse cursor as part of the screen content. This means that everytime you moved with your mouse cursor, we had to update whole image and that is very inefficient. The API in WebRTC allows you to implement MouseCursorMonitor which can be used to track mouse changes only and each platform can have both MouseCursorMonitor and DesktopCapturer implementations combined in DesktopAndCursorComposer to get complete image and this all works automatically like a magic. Unlike X11 implementation, our only option is to get everything from one PipeWire stream we connect to and there was no way how to make it shared from DesktopCapturer implementation so it can be used by MouseCursorMonitor implementation. I had to split DesktopCapturer to have xdg-desktop-portal and PipeWire separate implementations. Code for PipeWire is now a SharedScreenCastStream class which is being shared through DesktopCaptureOptions. This is set of parameters associated with each capturer instance and luckily this is also passed to MouseCursorMonitor so we can have access to already initialized PipeWire stream and get the cursor data from there. Implement MouseCursorMonitor with SharedScreenCastStream was then piece of cake.
This should again significantly improve performance of screen sharing, because moving with a mouse over a static screen content doesn’t need full screen content update.
Last but not least, I’m now in touch with Google developers who help me to review all my changes and discuss with me the current state, issues I have, etc. on monthly meetings we have. The plan is to make this finally enabled by default, hopefully in the first half of this year. There are still some things that need to be solved before this is enabled and there is lot of work ahead, but things look promising.
Plans for the future:
Implement stream restoration
this will allow us to skip the second portal dialog and I already have plan in my head how to do this in WebRTC. This is currently only supported by xdg-desktop-portal-gnome and xdg-desktop-portal-kde lacks this functionality.
Improve UX of the Chromium screen sharing dialog
Write tests for all PipeWire/portal code in WebRTC
Even though WebRTC is used in Firefox, I mostly talk about Chromium, because Firefox doesn’t use most recent WebRTC and will need to pick all the changes I did or rebase to newer WebRTC in order to have them. Firefox also has PipeWire/Wayland screen sharing enabled by default and doesn’t have UX issues as there is no internal screen sharing dialog like in Chromium.
I hope all these changes will make your experience better and next time when you read a new blog post I will be informing you about end of this journey.
The original implementation was using mmap() to get the buffer content. This worked correctly for current Intel GPUs, but was terrifically slow on e.g. AMD GPUs. Proper solution is to use OpenGL context to get the content from buffer. However, there were many implementations using mmap() already, including WebRTC and we needed a way how to properly communicate between the server and the client that when the client advertises DMA-BUF support, it means it doesn’t use mmap() and goes through OpenGL context instead.
Here are some issues if you want to read about the details:
This all resulted into a completely different way how the communication between the consumer and the producer should happen in order to use DMA buffers for way faster and smoother screensharing support. Both sides are now required to query the list of all supported modifiers and add this as a new stream parameter, including flags that the modifiers are mandatory parameter, rest of stream parameters are kept as before so we can keep using other types in case DMA-BUFs are not supported by the producer. Once both sides matches their expectations, we can query whether the stream includes modifiers, based on that we know we can use DMA buffers, which we now properly open using OpenGL context, while we kept mmap() for MemFd buffer types as fallback. This will result into faster screensharing support in your web browsers.
Last but not least, I made screensharing even faster, regardless of buffer type we use. Originally when we received buffer from PipeWire, we copied it to a local variable so we can apply cropping and adjust the position and only after that we copied this adjusted content into a DesktopFrame, which each DesktopCapturer (a class representing screensharing implementation) is supposed to return and let it be displayed by the browser. That means we performed two copy operations for each frame. I improved this implementation and now we copy the PW buffer content directly to a desktop frame which we can return directly so one copy operation less than before. I didn’t do some exact measures, but simply running htop and comparing usage of top 5 processes when sharing a 4k screen I got:
Original result: 66%, 64%, 26% 23%, 10%
Updated result: 41%, 39%, 19%, 17%, 12%
I also have some other improvements on my TODO list, all of them should bring some additional optimizations and improvements. I will keep you informed once I have news to share with you.
Both changes have been merged into WebRTC, that means it should be in Chrome/Chromium 96 (released during November 2021).
In 2019, I started with my first contribution to WebRTC. This was all about screen sharing support on Linux Wayland sessions, using xdg-desktop-portal and PipeWire. Back then, it was quite simple, we only had PipeWire 0.2 and all portal backends supported only screen sharing (no window sharing). While this was relatively easy, it was not ideal as each screen sharing request involved two portal dialogs to get the screen content on the web page itself. For me it was a big success, because I made quite a significant contribution to such a big project, which is used by many people, and a project which is used by all modern web browsers.
At the beginning of 2020, the year everyone would like to erase from their memories, we got PipeWire 0.3 (with slightly different API) and later with xdg-desktop-portal-gtk and xdg-desktop-portal-kde (later this year) people were finally able to share application windows. Support for all of this was lacking in WebRTC, because back then those were not available. I wanted to tackle all issues at once, bring support for window sharing and get rid of the “dialog hell” with portals, which was even worse with the new window sharing capabilities in portal backends.
This is what the situation looks like. With each request to share a screen, you got the preview dialog from Chromium. This dialog consists from three pages. One is for screen sharing making one portal request, second one is for window sharing, which is another portal request, and the last one is just to allow you to share a web page you have opened. You had to confirm both portal dialogs, then confirm the Chromium dialog and finally you got one more portal dialog (ouch) to get the screen content on the web page itself.
I had a solution. I made all portal calls identified with an ID and shared this ID (portal call) in Chromium between both pages in the Chromium preview dialog and with the request made for the web page itself. With this solution we only had ONE portal dialog. This was a perfect solution (at least seemed to be). I started working on this at the beginning of this year, we exchanged many emails with people from Chromium UX team, because I wanted to do also some minor UI changes in the preview dialog. Unfortunately, those were rejected for consistency with all platforms. It was not a big deal and I submitted my changes for review, keeping UI as it was, just adding all necessary bits into Chromium and WebRTC to make it all work.
I wish to say things went smoothly since then, but the opposite is true. It took a while to get everything reviewed, but this is probably no surprise with this year being weird and many people working from home with less than ideal conditions. Anyway, few months passed away, I ended up rewriting my changes many times, not even counting hours I spent on it. This all resulted into me being obsessed with this change, it mattered to me so much to get it merged. I was constantly thinking about how to make it better, I was many times fixing issues in the evening (as reviewers were mostly US based), instead spending time with my family. It would be even better to waste my time with my beloved Playstation. This had really negative impact on my mental health and I realized this has to stop and I simply gave up, because I couldn’t continue this way and needed a break. I abandoned both changes (WebRTC and Chromium) and decided to just pick changes I will be able to successfuly upstream. I probably made my change too ambitious and complicated or maybe it’s just Chromium not being ready for this kind of change, because some tweaks were specific for my use-case. It’s also hard to say I wish upstream devs had helped me more, because there is so much to understand around Wayland, portals and PipeWire and way how it all works together.
Anyway, with a new start, without pressure after gaving up on the change, I picked the most important changes and submitted them separately. I was surprised now how smoothly this went and how fast those changes were upstreamed. Simply those changes were simple, understandable and easy to review. I didn’t gave up on fixing the “dialog hell” completely, I have some other ideas, but next time I will try to submit them step by step and will keep some distance and my free time.
And what are the changes you can expect in upcoming Chromium release in 2021?
Support for PipeWire 0.3
You can now build Chromium/WebRTC with both PipeWire 0.2 and Pipewire 0.3. There is a new “rtc_pipewire_version” option you can pass to your build configs.
Window sharing support
There is probably no description needed. You will be able to share application windows in case you don’t want to share whole screen.
Suppport for DmaBuf and MemFd buffer types
This should allow faster transfer of your screen content from your Wayland compositor, through PipeWire to your browser.
Less portal dialogs involved
If you look back into the screenshot I posted above, you can see there are two portal dialogs opened just for the Chromium preview dialog. I at least tried to reduce this to just one portal dialog. This was done by removing the page for window sharing, because the screen share request will already handle both screen and windows.
I think you can expect above mentioned changes in Chromium 89 and I hope you will at least appreciate some of these improvements even though I didn’t deliver everything I wanted to. Also, thanks to Martin Stránský from our Firefox team, you can expect all these changes to be also part of Firefox.
I recently got an email from a user asking me how to make all this work on Fedora. Problem is that unlike in old XServer sessions, there are certain things which need to be enabled first. There are also dependencies which need to be installed and services which need to be running. While most of the dependencies are automatically installed and services automatically activated, there still might be situations when this is not true, for example when switching from another desktop so it’s better to cover it all. This tutorial targets Fedora, but it can be probably used by any other distribution.
Both screen sharing and remote desktop work almost identically on Wayland, they both use portals as a communication tool between applications and compositor (in this case Mutter) to start the process of sharing and setup PipeWire stream (see below). While portals were primarily meant to be used by sandboxed applications (e.g. Flatpak) to get access to system (like files or printing) outside the sandbox, their design perfectly fits for Wayland usage too. In Fedora you should have portals automatically installed, they are represented by two separate packages, first is xdg-desktop-portal, which is the portal service communicating with sandboxed applications and with a backend implementation of portals, and the second package is the backend implementation, in our case xdg-desktop-portal-gtk. Both are DBus activatable, which means they are automatically started whenever application calls them. The reason why portals consist from two services is that there can be multiple backends, each one providing native dialogs for your desktop. For example you don’t get a gtk dialog to open a file in KDE Plasma session or you want a backend communicating with specific compositor (like in our case with Mutter).
The second important dependency is PipeWire. PipeWire is the core technology used for screen content delivery from the compositor to applications. This is done throught a PipeWire stream shared between the compositor and application. PipeWire should be automatically installed on your system, the package name is pipewire and it provides socket-based activation so you shouldn’t need to worry if it’s running or not.
Enabling screen sharing and remote desktop in Gnome
You don’t seem to do any additional step in order to make screen sharing work. However, you need to enable remote desktop (if you want to). Go and open gnome-control-center (Settings) and there go to Sharing section. There you should see this window when you click on Screen Sharing:
If you don’t have such option, make sure you have installed gnome-remote-desktop, because I’m not sure whether it’s installed by default. Allowing screen sharing will start a server instance which you can connect to from another computer, using vnc://linux.local or your_computer_ip:5900. You will most likely need to open a hole to your firewall, but the same you need to do for any other VNC server. I tried to connect with Vinagre and Krdc (KDE VNC viewer) and both worked for me, but I was unable to connect with Tigervnc (vncviewer) due to not maching security type.
Screen sharing in Firefox
Firefox in Fedora already comes with PipeWire support enabled and you don’t need to do anything special. You can test it for example with this testing page. The PipeWire support in Firefox unfortunately needs to be enabled during build by handmade changes, which is most likely not happening in other distributions, but from what I now there is an ongoing effort to make this configurable with a build option.
Screen sharing in Chromium/Chrome
Similar situation is with Chromium, where PipeWire needs to be also enabled during build, but it’s already configurable via a build option. In Fedora we have this enabled by default. Official Chrome builds are build with PipeWire support enabled as well. I should maybe mention that PipeWire support is in Chromium starting with version 74.
Unlike with Firefox, this support needs to be also enabled in runtime. You can do that with following chrome flag:
Then you should be all set to be able to share a screen or a window from your Chromium or Chrome.
Don’t get scared by higher number of dialogs for screen/windows selection you will get when sharing screen in your browser. We are aware of this annoying usability issue and hopefully we will manage to solve it one day. The reason why this happens is that every browser provide their dialog for screen/window selection and in both browsers these dialogs show previews of your selection. You need to select screen/window first in the portal dialog for the preview dialog and once you accept the preview dialog in your browser, you again need to select screen/window in the portal dialog to get the content into the web page itself.
Support in other applications
There is a KDE application called Krfb, which in the next release (19.08) will have similar support for remote desktop on Wayland as you have in Gnome. Otherwise there are probably no other applications which would allow you to share a screen or control your desktop remotely on Gnome Wayland sessions. You will not be able to use TeamViewer, Tigervnc or any other application you are used to use. If you want to use these applications, you will have to switch to X session for now.
We already had WireGuard support in Plasma 5.15, but it existed as a VPN plugin based on a NM WireGuard plugin, which wasn’t really working very well and didn’t utilize many of already existing NM properties. With release of NetworkManager 1.16, we have a new native support of WireGuard which is much more usable. It now exists as a new connection type so it’s implemented a bit differently compared to other VPNs. This mean that we had to implement first support for this connection type and its properties into NetworkManagerQt and implement an UI on top of that. The UI part of the new WireGuard support, same as the old VPN plugin, were implemented by Bruce Anderson. We are also probably (at this moment) the only one who provides an UI for WireGuard configuration so thank you Brucefor such a big contribution.
OTP support in Openconnect VPN plugin
Another big contribution, this time made by Enrique Melendez, is support for one time passwords in the Openconnect VPN plugin. This support was missing for some time so starting with Plasma 5.16, you should be able to use TOTP/HOTP/RSA/Yubikey tokens for your Openconnect connections.
PAN GlobalProtect VPN
OpenConnect 8.00 introduced support for PAN GlobalProtect VPN protocol. You can now see this new VPN type entry thanks to Alejandro Valdes.
Remote desktop portal
Remote desktop portal brings possibility to control remotely your Wayland Plasma sessions. It utilizes screensharing portal to get the screen content and adds API for mouse/keyboard/touch control. Unfortunately at this moment only mouse support is implemented, mainly because I use KWayland::FakeInput protocol and mouse support is the only one currently implemented there. At this moment there is no Qt/KDE based application using remote desktop portal (or at least released one), but I have added support into Krfb, which is currently on review and I hope to get it merged for KDE Applications 19.08. Alternatively you can use gnome-remote-desktop.
Two days ago I wrote about our work on screen sharing in web browsers. While there was a lot of work done recently on this area, it’s not still in the state where everything would just work out of the box. There are few steps necessary to make this work for you and here is a brief summary what you need. This is not a distro specific how to, but given I use Fedora 28 and I know that everything you need is there, it’s most likely you will need to figure out the differences for your distribution or build it yourself.
PipeWire is the core technology used behind all of this. In Fedora you just need to install it, it’s available for Fedora 27 and newer. Once PipeWire is installed, you can just start it using “pipewire” command. If you want to see what’s going on, you can use “PIPEWIRE_DEBUG=4 pipewire” to start PipeWire with debug information. For Fedora 29, there is a feature planned for PipeWire which should make it to start automatically.
Xdg-desktop-portal and xdg-desktop-portal-[kde,gtk]
We use xdg-desktop-portal (+ backend implementation) for communication between the app requesting to share a screen and between desktop (Plasma or Gnome). You need xdg-desktop-portal, which is the middle man between the app and backend implementation, compiled with screencast portal. This portal will be build automatically when PipeWire is present during the build. In Fedora you should be already covered when you install it. For backend implementation, if you are using Plasma, you need xdg-desktop-portal-kde from Plasma 5.13.x, again compiled with screencast portal, which is build when PipeWire is present. For Fedora 28+, you can use this COPR repository and you are ready to go. I highly recommend using Plasma 5.13.2, where I have some minor fixes and if you have a chance, try to compile upcoming 5.13.3 version from git (Plasma/5.13 branch), as I rewrote how we connect to PipeWire. Previously our portal implementation worked only when PipeWire was started first, now it shouldn’t matter. If you use Gnome, you can just install xdg-desktop-portal-gtk from Fedora repository or build it yourself. You again need to build screencast portal.
Enabling screen sharing in your desktop
Both Plasma and Gnome need some adjustments to enable screen sharing, as in both cases it’s an experimental feature. For Gnome you can follow this guide, just enable screen-cast feature using gsettings. For Plasma, you need to get KWin from Plasma 5.13.x, which is available for Fedora in the COPR repository mentioned above. Then you need to set and export KWIN_REMOTE=1 env variable before KWin starts. There is also one more thing needed for Gnome at this moment, you need to backport this patchto Mutter, otherwise it won’t be able to match PipeWire stream configuration with the app using different framerate, e.g. when using Firefox.
Edit: It seems that exporting KWIN_REMOTE=1 is not necessary, it probably was only during the time when this feature was not merged yet. Now it should work without it. You still need KWin from Plasma 5.13.
Start with screen sharing
Now you should be all set and ready to share a screen on Gnome/Plasma Wayland session. You can now try Firefox for Fedora 28 or Rawhide from this COPR repository. For Firefox there is a WebRTC test page, where you can test this screen share functionality. Another option is to use my test application for Flatpak portals or use gnome-remote-desktop app.
Edit: I didn’t realize that not everyone knows about xdg-desktop-portal or PipeWire, below are some links where you can get an idea what is everything about. I should also mention that while xdg-desktop-portals is primarily designed for flatpak, its usage has been expanded over time as it perfectly makes sense to use it for e.g. Wayland, where like in sandbox, where apps don’t have access to your system, on Wayland apps don’t know about other apps or windows and communication can by done only through compositor.
Last time I wrote about possibility to share a screen of Plasma wayland session, using xdg-desktop-portal and our xdg-desktop-portal-kde backend implementation. Problem was that during that time, there was no application which would implement support for this, leaving my previous effort useless so far. Luckily, this should change pretty soon. I, together with my Red Hat collegues Tomáš Popela and Eike Rathke, have been working for past few weeks on bringing support for screen sharing on Wayland to web browsers. All modern browsers use WebRTC for all audio-video communication, including screen sharing, meaning that in a perfect world, just one implementation would be needed, which is not that exactly this case. Let’s go a bit into the details first.
Each system (Windows, Mac and X11) in WebRTC reimplements an abstract class called DesktopCapturer, which defines API used by applications to support screen sharing. For our wayland support, we started with a new implementation using Pipewire as the core technology used for screen content delivery and for the communication part, to request which screen to share and to obtain Pipewire stream information, we use xdg-desktop-portal, providing simple API to do so. Advantage of using xdg-desktop-portal is that it will work also in sandbox (Flatpak and Snap) and that there is support in Plasma (using xdg-desktop-portal-kde backend) and support for Gnome (using xdg-desktop-portal-gtk backend), both using same API. Using our desktop capturer implementation, WebRTC starts to communicate with xdg-desktop-portal, we set up a session associated with our request, we tell xdg-desktop-portal that we have interest in screen sharing so xdg-desktop-portals asks your backend implementation to provide a dialog to select a screen to share and once this is settled, we request to start screen sharing and backend implementation will set up Pipewire stream, sending us back file descriptor of a Pipewire remote which we can open and connect to it. Once we are connected, we finally start receiving buffers from Pipewire with screen data and providing them to applications. This so far sounds simple and that the work is basically done, but this is unfortunately not an ideal world.
We found out that e.g. Firefox uses some older copy of WebRTC, so while working on WebRTC trunk, to have our work ready for upstream, we had to modify slightly our changes for Firefox older copy in order to be able to test our changes. There is also one thing we haven’t figured out yet, the thing is that Firefox has its own dialog to select a screen (for other capturer implementations) and we were not able to avoid displaying it when our capturer is used. Another thing is Chromium, which seems to be using WebRTC in a different way when compared to Firefox as Chromium is using plugins so this is also something we have to figure out. There are probably still plenty of other things to do before all of this can be upstream, but we have made great progress on this so far. We even had couple of Bluejeans calls where both sides could share their screens, one running Gnome Wayland session and me running Plasma Wayland session.
And for those who like adventures, I have set up a Fedora COPR repository (currently building), with Firefox containing our changes so you can test it yourself, we will need testers soon or later anyway. You just need to make sure that Pipewire is running, otherwise this won’t work, portals (both xdg-desktop-portal and backend implementation) should be started automatically.
One of the important missing features in Plasma wayland session is without a doubt possibility to share your screen or record you screen. To support this you need help of the compositor and somehow deliver all needed information to the client (application), in ideal way something what can be used by all DEs, such as Gnome. Luckily, this has been one of the primary goals of Pipewire, together with support for Flatpak. If you haven’t heard about Pipewire, it’s a new project that wants to improve audio and video handling in Linux, supporting all the usecases handled by PulseAudio and providing same level of handling for video input and output. With Pipewire supporting this, there was recently a new API added to xdg-desktop-portal for screen cast support and also for remote desktop. Using this API, applications can now have access to your screen content on Wayland sessions or in case they are running in sandbox. With various backend implementation, like xdg-desktop-portal-kde or xdg-desktop-portal-gtk, they just need to support one API to target all desktops. Screen cast portal works the way, that the client first needs to create a session between him and xdp (xdg-desktop-portal) backend implementation, user then gets a dialog with a screen he would like to share and starts screen sharing. Once he does that, xdp backend implementation creates a Pipewire stream, sends back response to the client with stream id and then client can connect to that stream and get its content. Once he no longer requests content of the selected stream, xdp backend implementation gets information that nobody is longer connected to the created Pipewire stream and can stop sharing screen information and xdp backend implementation is again ready to accept next requests for screen sharing. This is all happening in the background so there is really no cool picture I can show, at least this dialog which you get when you request to share a screen.
I finished support for screen cast portal in xdg-desktop-portal-kde last week and currently waiting for it to pass review and be merged to master. This is also currently blocked by two not merged reviews, one adding support for sending GBM buffers from KWin and one with new Remote Access Manager interface in KWayland, both authored by Oleg Chernovskiy, for which I’m really greatful. This all will hopefully land soon enough for Plasma 5.13. Testing this is currently a bit complicated as you need everything compiled yourself and besides my testing application there is really no app using this, except maybe Gnome remote desktop, but there should be support in future for this in Krfb, Chrome or in Firefox. Hopefully soon enough.
Last thing I would like to mention is for GSoC students. We also need remote desktop portal support to have full remote desktop experience so I decided to propose this as a GSoC idea so students can choose this interesting stuff as their GSoC work.