The main window height was always bigger than my screen height on
startup, overriding my previous session's window size.

I could retrace the memorized size being changed when adding the display
shell with gimp_image_window_add_shell() in gimp_display_new(), just
before showing the window. Unfortunately this happens after we applied
the session position/size (in gimp_image_window_session_update() at end
of image window construction). I'm not sure why adding the shell
increases the size of the window, especially since the window can be
manually sized at the expected dimension without any graphical glitch.

Maybe we could investigate this, but simply forcing any session managed
window to behave as expected upon showing is not a bad move anyway and
in this specific case, it works fine.