bug#38181: Actual height of mode-line not taken into account

[martin rudalics]

> Now, let's define test-popup-no-redisplay as the version of test-popup
> with the call to redisplay removed, and test-popup-redisplay as the
> version that calls redisplay before fitting the window to its buffer:
>
> (test-popup-no-redisplay) -> cropped content
> close popup
> (test-popup-no-redisplay) -> cropped content
> close popup
> (test-popup-redisplay) -> ok
> close popup
> (test-popup-redisplay) -> ok
> close popup
> (test-popup-no-redisplay) -> cropped content
> close popup
>
> As you can see, the redisplay only "fixes" the next fit, after that the
> problem reappears.

Right.  The "close popup" removes anything redisplay cached for the
window's mode line in the last call of 'test-popup-redisplay'.

> This may be of interest:
>
> (test-popup-no-redisplay) -> cropped content
> (test-popup-no-redisplay) -> ok
> (test-popup-no-redisplay) -> ok
> (test-popup-no-redisplay) -> ok
>
> (N.B. I'm not closing the popup) It seems like the window was reused
> and the second time it got things right.

Right.  It always uses the cached mode line height in this case.

> My interpretation is that the early redisplay "prepares" the current
> window for the fit. Now, the workaround that I linked above does a
> redisplay once per buffer, not once per window.

What is the "workaround"?  IIUC there is no "redisplay" one can call on
a "per buffer" base.

> The issue I observe,
> which I believe is the same one that motivated this report in the
> first place, is an org-set-tags-command menu clipped at the bottom
> (it's not the only case, though). Since the popup windows that
> org-mode opens for this and other menus are transient but their
> buffers likely remain hidden, that may be the reason why the
> workaround is not, well, working around. What I'm failing to grasp is
> how could it ever work... maybe there was some change in the
> implementation of org-mode popups.
>
> I would be happy with a sound statement like "if you change the
> modeline height so it is different to the default char size, you need
> to call redisplay for each window that sports the modified modeline
> before executing any operation that requires knowledge of the geometry
> of that modeline, including fit-window-to-buffer". If that's true then the
> current trick could be reasonably modified to cope with every possible
> case instead of hooking to particular functions (fit-window-to-buffer) in
> maybe the wrong scope (buffer), by just triggering an early redisplay
> each time a new window is created.

Even the 'redisplay' trick will not be sufficient: Consider Eli's
scenario in this thread but with _different_ heights for active and
inactive mode lines.  It will break things when after doing the
'fit-window-to-buffer' call you (de-)select the window you've just fit.

> Rereading (some of) the above thread, I noticed you said:
>
>> If we don't want 'fit-window-to-buffer' to do that always we'd need
>> some variable, either buffer local or even a window parameter, that
>> 'fit-window-to-buffer' would inspect once and reset immediately in
>> order to perform only the redisplay call that's really needed.
>
> I believe this is similar to what I've just suggested. Then a long
> discussion full of technicalities that I won't be able to follow in
> depth anytime soon took place. I understand that maybe forcing an
> early display is not ideal because, in principle, we only want to
> update the size of the modeline, not prematurely redisplay the window.
> Moreover, modelines could change after creating a window,
> but this is not an interesting use case in real life.
> But since creating a window is not a frequent operation, and
> changing a modeline on the fly is not a likely event, would it be
> so bad to trigger that early redisplay on window creation?

I think you mean with every set_window_buffer?  And probably with every
set_window_configuration too.  Did you try it?

> I'm not saying that emacs should do it, I'm confident that your
> decision in this regard will be far better than anything I could
> come up with, I'm just looking for a workaround that gets the job
> done or, alternatively, the certainty that it's a bad idea and that we
> should refrain from pretiffing modelines.

Here I have no problems with the scenarios I've seen in this thread
because I update the mode line caches with every change of a window
height (which includes the case where a window is created) and with any
change of a buffer local variable like 'mode-line-format'.  So basically
no redisplay is ever needed in the first place, just a recalculation of
the mode line heights of all windows whose heights have changed.

martin