GTK4 on X11 fails to respond correctly to mouse movements with intel drivers, with freezes and lagging
The X11 backend of GTK4 freezes and fails to respond correctly to mouse movements under mutter/gnome-shell. This affects gtk4-demo and any other GTK4 program under mutter. The windows of any GTK4 applications cannot be dragged correctly (there is either no response or a very delayed response), and clicking on widgets such as buttons and menus fails to have an effect or the effect is much delayed. It is as if a significant portion of mouse events under X11 are being dropped. Keyboard events seem mostly to work OK.
This occurs with both the modesetting X11 driver and the (now superceded) xf86-video-intel driver.
GTK4 works fine under Xorg with the XFCE window manager or under window maker, and it also works fine under mutter/gnome-shell with the wayland backend. So it is specific to the X11 backend only of mutter/gnome-shell.
Issues #1269 and #1583 (closed) may be related to this, but the problem is much more general and severe than the issue reported in #1269: the issue occurs with any GTK4 window, not just fishbowl/gears, and prevents you even starting gtk4-demo's fishbowl or gears without a struggle.
Edit: I am using mutter-3.38.3, xorg-server-1.20.10 and mesa-20.3.4.