The wording of the xdg-shell protocol allows surfaces to not cover the whole screen when they are made fullscreen. From the description of the fullscreen state in xdg-shell:
The window geometry specified in the configure event is a maximum; the client cannot resize beyond it. For a surface to cover the whole fullscreened area, the geometry dimensions must be obeyed by the client.
The last sentence is the condition for fullscreen coverage, not a requirement.
This commit updates the code to not flag size mismatches for fullscreen surfaces as a protocol error when the surface fits within the screen. In such cases, the shell is responsible for centering surfaces appropriately and also for obscuring other screen content as described in the xdg_toplevel.set_fullscreen request description (and, indeed, desktop-shell does all this).
For reference, contrast with the corresponding, stricter wording in the obsolete xdg-shell-unstable-v6 protocol for the fullscreen state:
The window geometry specified in the configure event must be obeyed by the client.
f6bd2129 xdg-shell: Allow fullscreen surfaces to not cover the whole screen
libweston-desktop/xdg-shell.c | 22 ++++++++++++++++++++--
1 file changed, 20 insertions(+), 2 deletions(-)