The accurate description of this option was:
- fit-border is enabled by default. When disabled, it adds a bug where
if the window has borders and mpv shrinks it to fit the desktop, then
the calculation ignores the borders and adds incorrect video crop.
The option was added at commits 70f64f3c and 949247d6, in order to
solve an issue (#2935) where if mpv wanted to display a video with
size WxH, then w32_common.c incorrectly set the window to WxH, while
down-scaling the video slightly to fit (even with small sizes).
It was addressed with a new option which is enabled by default, but
does the right thing (sets the client area to WxH) only when disabled,
so that everyone who prefers their video slightly downscaled could
keep their default behavior.
(#2935 also addressed an off-by-one issue, fixed before fit-border)
While disabling the option did avoid unnecessary downscaling, it also
added a bug when disabled: the borders are no longer taken into
account when the size is too big for the desktop. Most users don't
notice and are unaffected as it's enabled by default.
Shortly later (981048e0) the core issue is fixed, and now the client
area is correctly set to WxH instead of the window (and together with
the three following commits which center the video, adds a new bug
where the window title can be outside the display - addressed next).
However, fit-border remained, now without any effect, except that it
still has the same bug when disabled and the window is too big.
Later code changes and refactoring preserved this issue with great
attention to details, and it remained in identical form until now.
Simply rip out fit-border.
ef1d0b2cdb