Wayland: broken splash (size and position)
GIMP Version: master
Operating System: Wayland/Linux
The size issue is because
gdk_monitor_get_workarea() returns unscaled display size which makes it twice the size for instance with scale ratio ×2, even though GDK docs clearly says that it should return scaled size (and that's what it does on X11). I opened a bug report with some initial patch attempt about this:
Unfortunately I am told that there is no way to get the right value on Wayland and that this API should not be relied on (basically it mostly returns some guess but it is not trustworthy). The real solution would be to create a "splash" window role (this doesn't exist yet), so that Wayland knows what to do with it. There may be some workaround for GTK+3 (to be continued).
In any case, the solution is likely not in GIMP code, but I open this report for tracking the issue.
The position issue is that we can't set absolute positions of our windows anymore on Wayland (we can set relative positions of child dialogs, but not absolute position in the screen of the toplevel windows), as confirmed by @carlosg on IRC. Not sure what would be the right solution, but probably a "splash" role would also give a neat solution.