Improving Xwayland window resizing

Speaking of Wayland, one of the most important parts of the transition is Xwayland, which makes sure legacy X applications not yet capable of running under a modern graphics stack can continue to function. Xwayland applications have had this weird visual glitch during resize operations, however, where the opposite side of the window would expand and contract while resizing. KDE developer Vlad Zahorodnii wanted to fix this, and he wrote a very detailed article explaining why, exactly, this bug happens, which takes you deep into the weeds of X and Wayland.

Window resizing in X would be a glitchy mess, if it wasn’t for the X11 protocol to synchronize window repaints during interactive resize, which ensures that the window resize and the application repainting its window contents remain synchronised. This protocol is supported by Kwin and GNOME’s Mutter, so what’s the problem here? Shouldn’t everything just work?

KWin supports the basic frame synchronization protocol, so there should be no visual glitches when resizing X11 windows in the Plasma Wayland session, right? At quick glance, yes, but we forget about the most important detail: Wayland compositors don’t use XCompositeNameWindowPixmap() or xcb_composite_name_window_pixmap() to grab the contents of X11 windows, instead they rely on Xwayland attaching graphics buffers to wl_surface objects, so there is no strict order between the Wayland compositor receiving an XSync request acknowledgement and graphics buffers for the new window size.
↫ Vlad Zahorodnii

Basically, the goal of the fix is to make sure these steps are also synchronised when using Xwayland, and that’s exactly what Zahorodnii has achieved. This makes the resizing X windows under Xwayland look normal and without weird visual glitches, which is a massive improvement to the overall experience of using a Wayland desktop with a few stray X applications. Thanks to this fix, which was made possible with help from Wayland developers, Kwin is now one of the few compositors that correctly synchronises X windows running under Wayland.

KDE has been doing an amazing job moving from X to Wayland, and I don’t think there’s anyone else who has managed to make the transition quite as painless. Not only do KDE developers focus on difficult bugs like this one that many others would just shrug off as acceptable jank, they also made things like the Wayland to X11 Video Bridge, a desktop-agnostic tool to allow things like screen sharing in Teams, Discord, Slack, etc. to work properly on Wayland.