OK.  Eli, I would like to install the attached patch for Emacs 27.  All
changes affect the resizing and moving of child frames only, normal
frames are not affected.  The Change Log below tries to explain why they
are needed.  One change (the addition of Fx_gtk_debug) is not needed but
might be useful in order to debug GTK issues from inside Emacs if and
when we receive more bug reports in this area.

martin


Fix problems with moving and resizing child frames

(1) Re-implement 'mouse-drag-frame' via two new functions
'mouse-drag-frame-resize' and 'mouse-drag-frame-move'.  This is
needed because with some toolkits the notifications for frame
movement and resizing arrive asynchronously, breaking any
calculations using intermediate frame sizes and positions.
(2) Provide new option 'x-gtk-resize-child-frames' which allows
to either hide a child frame during resizing or asks GTK to
resize it "immediately".  This is needed because desktops like
GNOME shell otherwise won't allow resizing child frames at all.
(3) Do not try to synchronize the position of a child frame
after moving it.  Needed because the present implementation
introduces a 0.5 secs delay which makes dragging child frames
virtually impossible with Lucid and Motif toolkits on desktops
like GNOME shell that use invisible outer frame borders.

For further information see the thread starting with
https://lists.gnu.org/archive/html/emacs-devel/2020-01/msg00343.html

* lisp/mouse.el (mouse-drag-mode-line, mouse-drag-left-edge)
(mouse-drag-top-left-corner, mouse-drag-top-edge)
(mouse-drag-top-right-corner, mouse-drag-right-edge)
(mouse-drag-bottom-right-corner, mouse-drag-bottom-edge)
(mouse-drag-bottom-left-corner): Call 'mouse-drag-frame-resize'
instead of 'mouse-drag-frame'.
(mouse-drag-frame): Split into two new functions -
'mouse-drag-frame-move' and 'mouse-drag-frame-resize'.
(mouse-drag-frame-resize, mouse-drag-frame-move): New functions
to implement functionality of the removed 'mouse-drag-frame'.
* src/frame.c (syms_of_frame): New symbol Qxg_frame_set_char_size_4
* src/gtkutil.c (xg_frame_set_char_size): Hide frame
during resizing when 'x-gtk-resize-child-frames' equals 'hide'.
* src/xfns.c (x_set_parent_frame, Fx_create_frame): Set
gtk_container_resize_mode to GTK_RESIZE_IMMEDIATE
when'x-gtk-resize-child-frames' equals 'resize-mode'.
(Fx_gtk_debug): New function to toggle interactive GTK debugging
from within Emacs.
(syms_of_xfns): New symbols Qhide and Qresize_mode.
(x-gtk-resize-child-frames): New option that allows to resize
child frames on desktops like GNOME shell (with the mutter WM)
that generally refuse to resize them.
* src/xterm.c (x_set_offset): Don't x_sync_with_move for child
frames, it makes moving child frames virtually impossible with
the Lucid and Motif toolkits.