I ran this as emacs -Q -l repro-fullheight.el and it seems to work identically for me with and without pixelwise and implied resize.

(setq frame-resize-pixelwise t)
(setq frame-inhibit-implied-resize t)
(set-frame-parameter nil 'fullscreen 'fullheight)

Should I be doing this differently?

On Thu, Jan 9, 2025 at 3:52 AM martin rudalics <rudalics@gmx.at> wrote:
 > Just got around to testing fullheight on GTK. Under emacs -Q, I do not see
 > a gap either on 29.4 or master from a week ago. This is on Debian 12 with
 > GTK 3.

Did you try with 'frame-inhibit-implied-resize' non-nil (ideally set in
an early-init file)?

martin