bug#64596: 30.0.50; On FIXME: in src/buffer.c:1481 (force-mode-line-update)

[Stefan Monnier]

>> The actual issue with the above is different: it implies that
>> update_mode_lines _always_ indicates that more than a single mode line
>> should be updated (which is why we need to "consider all windows").
>> But is that actually true, i.e. does every place that assigns non-zero
>> to update_mode_lines indeed perform a change which makes it
>> _necessary_ to consider non-selected windows?
>
> At least, when the current buffer is only displayed in a single,
> selected window, checking every window should not be necessary.

Indeed, we have a special case for that.  Before my change, it was
either "just the selected window" or "all windows".

> Compare bset_redisplay and bset_update_mode_line.

Indeed the old code updated all windows after a call to
`force-mode-line-update` even if the current buffer is only displayed in
the selected window and I somewhat preserved that :-(


        Stefan