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bug#44933: 27.1; Ephemeral frame selection shrinks minibuffer

From: martin rudalics
Subject: bug#44933: 27.1; Ephemeral frame selection shrinks minibuffer
Date: Mon, 30 Nov 2020 10:04:54 +0100

> In my reproduction, the minibuffer window is grown manually via
> `window-resize`.

With 'resize-mini-windows' non-nil this usually has no visible effect
because any such manual resizing is immediately undone by the automatic
resizing scheme.

> In the case of selectrum, the completing read
> candidates are displayed via overlay. I think this means that a
> resizing function that considers the buffer text only will resize
> these back down to a single line.

The routine that determines the window size (resize_mini_window in
xdisp.c) should consider overlays as well (if they are in the right
buffer).  Anything else would be a bug.

> I just tested, and binding `resize-mini-windows` to nil around the
> window selection in `term-emulate-terminal` does solve my
> problem. Does this seem like the right fix? I think it is surprising
> that output from a term process coming in the background should resize
> a minibuffer window (and especially an active one).

If you look into the code of do_switch_frame (in frame.c) you will be
able to spot

  if (!for_deletion && FRAME_HAS_MINIBUF_P (sf))
    resize_mini_window (XWINDOW (FRAME_MINIBUF_WINDOW (sf)), 1);

which means that currently even redisplay itself may resize the mini
window every time it constructs a mode line or frame title.  I can only
offer two advices: (1) Avoid 'with-selected-window/frame' in timers and
(2) make sure no redisplay happen _within_ such a form.

> Also, I see that `term-emulate-terminal` is calling `select-window` to
> perform its window selections. From my reading of the docs, I think it
> might make sense for it to pass `mark-for-redisplay` as the `norecord`
> argument. It doesn't seem like we should be modifying the buffer list
> or most recently selected window in this case, but we do want to
> redisplay the new output.

I would have to understand the semantics of 'term-emulate-terminal' to
answer that.  Hopefully, someone else can chime in here.

> I think the documentation should clarify that window selection can
> change window sizes as a side effect. The docs for selecting windows
> do not mention that `select-window` can change minibuffer sizes. The
> docs for minibuffer windows
> do mention these variables to control how minibuffer windows can be
> resized automatically, but they do not say what functions might try to
> do this automatic resizing.

With the recently added 'minibuffer-follows-selected-frame' we now have
an additional source of complications to consider.  Maybe you could, as
soon as the implementation of the latter has consolidated, play with the
various values of 'resize-mini-windows' and suggest suitable fixes for
the documentations of 'select-window' and 'select-frame'.

Alternatively, we could consider skipping the

    resize_mini_window (XWINDOW (FRAME_MINIBUF_WINDOW (sf)), 1);

above for temporary window/frame selections (and at least for redisplay
as I currently do here) but the consequences of such a step are not
easily fathomable.

> Finally, I wonder if `with-selected-window` (which
> `term-emulate-terminal` does not currently use) should bind
> `resize-mini-windows`.

... with the consequences I mentioned in the sentence before ...

> The docs say it is "the preferred way to
> temporarily work with" a selected window, so it does seem like
> automatic resizing is not in the spirit of the function. On the other
> hand, I don't know all the existing use cases; maybe this would break
> things.



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