One of the greatest annoyances after switching my work systems to Wayland is the lack of clipboard synchronization between the X11 and Wayland windowing system. X11 clipboard is only available to X11 windows and the Wayland clipboard is only available to Wayland windows. It’s the useability problem of the two X11 clipboard (primary and secondary) just exposed on a much higher and more annoying way. Instead of “just” remembering whether you copied or selected text, you need to remember which windowing system you copied from and which windowing system the current window is in. And if they are not in the same, there is just no way.

So getting this problem resolved got quite a priority. It was something I had never really spent any thought on and incorrectly assumed it’s something Xwayland would take care of. But after thinking about the problem I realized that the X11 clipboard is part of ICCCM and not a core feature of the X-Server. It’s a communication protocol between clients.

This means that KWin had to learn to communicate with the X11 clipboard in order to provide the synchronization. Not an easy task as ICCCM is quite complex on this topic and implementing an X11 clipboard is nothing I’m looking forward to. Luckily I remembered that we have a clipboard manager for X11 (klipper) and that this one works without having X11 specific code thanks to Qt’s abstraction.

And this makes the whole thing way easier: by using Qt’s X11 clipboard implementation we only need to interact with Wayland’s clipboard which is thanks to KWayland quite straight forward. The solution involves a small helper binary which gets started by KWin. It’s forced to the xcb windowing system to be able to use Qt’s X11 clipboard and manually connects with KWin through KWayland. The task of the binary is quite simple: whenever the X11 clipboard changes it gets the data and sets it on the Wayland clipboard. Similar whenever the Wayland clipboard changes it gets set on the X11 clipboard.

Unlike the X11 clipboard on Wayland clients are normally not informed about clipboard changes. Only the application currently having keyboard focus gets informed by the Wayland server about what kind of data is available on the clipboard. This means we don’t need to inform all Wayland clients about changes in the X11 clipboard and similar we don’t need to inform X11 about clipboard changes in Wayland all the time. Instead KWin helps the process: when an X11 window gains keyboard focus our helper process gets informed about the current Wayland clipboard content, so that it can be available for the focused X11 window and while X11 windows are focused clipboard changes from the helper application are forwarded to Wayland. So when the next Wayland window gets focused it has the clipboard content set on X11. A small and simple solution which works quite well in practice. At the moment only the “real” clipboard is synchronized. There is not yet the concept of the selection based clipboard on Wayland. A solution might be to just synchronize it to the normal clipboard. But this would again introduce the useability problems of this non-explicit clipboard. So I’m not yet convinced what’s the proper way.

In my last blog post I pointed to our current donation campaign to support the Randa developer sprint. A lot of readers donated after reading this. I want to thank everybody who donated. It’s so important for a free software community to meet in person and we need donations for getting developers to sprints and conferences. Last week I was at openSUSE conference and had a talk about how Wayland improves the quality of Plasma. These are important things to talk about at a conference, but it’s only possible if we have the donations to send people there. Please help to make such developer sprints possible. Thank you!