bug#31968: 26.1.50; Allow to hide title bar on maximize (gtk/gnome/csd)

[martin rudalics]

> One simple possibility is to set/unset
> the _GTK_HIDE_TITLEBAR_WHEN_MAXIMIZED property. I think there is also a
> corresponding gtk api for that but maybe the property could be set using
> standard X machinery already exposed to elisp(?). Probably all this would
> be ineffective under waylan but it's simply enough to fix the problem for X
> with low effort.

Could someone try to add a 'xg_set_hide_titlebar_when_maximized'
function to gtkutil.c.  That would be just like 'xg_set_undecorated'
but call 'gtk_window_set_hide_titlebar_when_maximized' instead of
'gtk_window_set_decorated'.  The Lisp interface for the moment could
be a simple 'x-set-hide-titlebar-when-maximized' function with a FRAME
and HIDE argument defined in xfns.c for GTK only that calls
'xg_set_hide_titlebar_when_maximized' with a frame structure
corresponding to the frame argument.

'gdk_x11_window_set_hide_title_bar_when_maximized' in gdkwindow-x11.c
has the XChangeProperty calls to make this work for non-GTK builds.

martin