Problem: diff: using diff anchors with hidden buffers fails silently
Solution: Give specific error message for diff anchors when using hidden
buffers (Yee Cheng Chin).
Diff anchors currently will fail to parse if a buffer used for diff'ing
is hidden. Previously it would just fail as the code assumes it would
not happen normally, but this is actually possible to do if `closeoff`
and `hideoff` are not set in diffopt. Git's default diff tool "vimdiff3"
also takes advantage of this.
This fix this properly would require the `{address}` parser to be
smarter about whether a particular address relies on window position or
not (e.g. the `'.` address requires an active window, but `'a` or `1234`
do not). Since hidden diff buffers seem relatively niche, just provide a
better error message / documentation for now. This could be improved
later if there's a demand for it.
related: vim/vim#17615closes: vim/vim#17904cad3b2421d
Co-authored-by: Yee Cheng Chin <ychin.git@gmail.com>
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