www/firefox displays center square on fullscreen zoom

[Van Ly <van.ly%SDF.ORG@localhost>]

There are two ways to fullscreen zoom a clip for playback on firefox.

One way, the direct way, which is to click on the outward pointing
double arrows on the scrub bar, this leaves a square screen artifact at
the center, which was not there in earlier firefox versions on netbsd 11.

       https://sdf.org/~van.ly/img/firefox151_has_center_square_artifact_on_clip_playback.webp

The other way, the indirect way, uses the special hover
picture‑in‑picture control widget, that behaves as expected and has no
artifact on fullscreen zoom.

-- 
vl