On Tue, 3 Sept 2024 at 16:52, martin rudalics <rudalics@gmx.at> wrote:
 >> Reproduction with Emacs 29.3 (also with git master HEAD, see below).
 >>
 >> Run: emacs -Q
 >> C-x 5 2 ; window opens the same size as the initial window
 >> M-x menu-bar-mode RET ; disable menu-bar-mode

Here with a GTK-3 build on XFCE disabling menu-bar-mode makes both
frames smaller by the menu bar height.  Does this happen on your system?

Yes it does.

 >> C-x 52 ; window opens much smaller than initial window
 >>
 >> With master HEAD (commit 92ea393a16e), the situation is slightly different:
 >>
 >> Run: emacs -Q
 >> C-x 52 ; window opens much smaller than initial window

In all these cases after every single step please evaluate

(frame-geometry)

and post the results here.

Sure thing:

Emacs 29:

emacs -Q
(frame-geometry): ((outer-position 1127 . 81) (outer-size 1384 . 1504) (external-border-size 28 . 34) (outer-border-width . 0) (title-bar-size 0 . 46) (menu-bar-external . t) (menu-bar-size 1328 . 50) (tab-bar-size 0 . 0) (tool-bar-external . t) (tool-bar-position . top) (tool-bar-size 1328 . 82) (internal-border-width . 0))
C-x 5 2
menu-bar-mode RET
(frame-geometry): ((outer-position 1127 . 81) (outer-size 1384 . 1454) (external-border-size 28 . 34) (outer-border-width . 0) (title-bar-size 0 . 46) (menu-bar-external . t) (menu-bar-size 0 . 0) (tab-bar-size 0 . 0) (tool-bar-external . t) (tool-bar-position . top) (tool-bar-size 1328 . 82) (internal-border-width . 0))
C-x 5 2
(frame-geometry): ((outer-position 28 . 90) (outer-size 456 . 570) (external-border-size 28 . 34) (outer-border-width . 0) (title-bar-size 0 . 46) (menu-bar-external . t) (menu-bar-size 0 . 0) (tab-bar-size 0 . 0) (tool-bar-external . t) (tool-bar-position . top) (tool-bar-size 400 . 82) (internal-border-width . 0))

Emacs git master (commit 92ea393a16e):
emacs -Q
(frame-geometry): ((outer-position 28 . 90) (outer-size 1384 . 1504) (external-border-size 28 . 34) (outer-border-width . 0) (title-bar-size 0 . 46) (menu-bar-external . t) (menu-bar-size 1328 . 50) (tab-bar-size 0 . 0) (tool-bar-external . t) (tool-bar-position . top) (tool-bar-size 1328 . 82) (internal-border-width . 0))
C-x 5 2
(frame-geometry): ((outer-position 108 . 170) (outer-size 456 . 620) (external-border-size 28 . 34) (outer-border-width . 0) (title-bar-size 0 . 46) (menu-bar-external . t) (menu-bar-size 400 . 50) (tab-bar-size 0 . 0) (tool-bar-external . t) (tool-bar-position . top) (tool-bar-size 400 . 82) (internal-border-width . 0))

The window manager is mutter, I suppose?

Indeed, yes.

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