Firefox bug report: https://bugzilla.mozilla.org/show_bug.cgi?id=1866770 The issue is caused by sending a motion event outside popup bounds before an enter event for the next focused surface.
A possibly relevant merge request was started @ https://invent.kde.org/plasma/kwin/-/merge_requests/8064
Git commit 28fe2a786a18a5aad587d5d6ca93b466be4c0be9 by Vlad Zahorodnii. Committed on 02/09/2025 at 16:13. Pushed by vladz into branch 'master'. Fix dnd in Firefox bookmarks toolbar popups When the pointer crosses between popups, kwin will send a motion event for the old popup whose coordinates lie outside the corresponding surface followed by a leave and an enter event for another popup. That motion event confuses Firefox, it thinks that the pointer has left the popup so it should be closed. This change re-arranges the order of motion and enter/leave events so the latter come first. This is still not a great solution, a motion event will be sent even if an enter event has been sent, but it is good enough for the time being. M +4 -7 src/input.cpp M +2 -2 src/xwayland/databridge.cpp M +1 -2 src/xwayland/databridge.h M +2 -2 src/xwayland/dnd.cpp M +1 -3 src/xwayland/dnd.h M +1 -2 src/xwayland/drag.h M +2 -2 src/xwayland/drag_wl.cpp M +1 -2 src/xwayland/drag_wl.h M +5 -4 src/xwayland/drag_x.cpp M +1 -2 src/xwayland/drag_x.h M +3 -3 src/xwayland/xwayland.cpp M +1 -1 src/xwayland/xwayland.h M +1 -13 src/xwayland/xwayland_interface.h https://invent.kde.org/plasma/kwin/-/commit/28fe2a786a18a5aad587d5d6ca93b466be4c0be9