[Firefox on Xorg] When system titlebar is disabled, Firefox window can't be resized by mouse

This is mozilla bug https://bugzilla.mozilla.org/show_bug.cgi?id=1530252

Firefox uses shapes to draw transparent titlebar corners. Mutter fails to resize the Firefox window by mouse when the shape mask is applied to Firefox main window. Happens only in X.org, Xwayland works ok. Can be reproduced on other applications which uses shapes (xeyes for instance).

Reproduction steps:

1. run mutter on Xorg, not XWayland
2. launch xeyes
3. try to resize the window, left and bottom window edge can't be snapped by mouse
4. the window itself is difficult to move by mouse