[Firefox] When GtkHeaderBar is removed from toplevel window some parts of the window do not get mouse events

This is mozilla bug https://bugzilla.mozilla.org/show_bug.cgi?id=1527315

When titlebar rendering is turned off (for instance print preview is activated) some parts of Firefox window are not clickable (buttons, scrollbars). Happens only in CSD mode. There's a simple testcase attached:

gtk-empty-titlebar.cpp

0. build it
1. run gtk-empty-titlebar, you'll get empty window without the titlebar (that's intentional as Firefox draws it's own titlebar)
2. click anywhere at the window. CSD mode will be disabled and window manager decorations will be enabled
3. enlarge the window or make it fullscreen
4. click at the new window area which was added in step 3)
5. the new area is not clickable. Only the old window parts which was there before step 3 are clickable.