bug#47498: 28.0.50; follow-mode misses a line with tiling window manager

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bug#47498: 28.0.50; follow-mode misses a line with tiling window manager

Eli Zaretskii
> From: Stefan Kangas <[hidden email]>
> Date: Tue, 30 Mar 2021 12:30:42 +0000
>
> I see one hidden line when using follow-mode.
>
> Steps to reproduce:
>
> 0. emacs -Q
> 1. C-h i
> 2. C-x 2
> 3. M-x follow-mode RET
>
> There is now one line missing from the display.  When using
> `display-line-numbers-mode', this effect is more obvious.

From my POV, follow-mode's design is fundamentally flawed, when
variable-size fonts and fractional line display are considered.  It
attempts to solve several problems that cannot be solved in principle,
unless the only font on display is the default face's font, and the
windows display an integral number of text lines.  It's a small wonder
in my eyes that display problems caused by follow-mode are so minor,
even subtle.  My advice is to stay away of it, if you want perfect
display.



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bug#47498: 28.0.50; follow-mode misses a line with tiling window manager

Eli Zaretskii
> Date: Tue, 30 Mar 2021 15:56:00 +0300
> From: Eli Zaretskii <[hidden email]>
> Cc: [hidden email]
>
> From my POV, follow-mode's design is fundamentally flawed, when
> variable-size fonts and fractional line display are considered.

Oh, and let's not forget features like display properties and
overlays, which can disrupt the display continuity if they happen on
the last line of the left window and continue into the first line of
the right one.  That's because follow-mode works by forcing the
window's start point, but Emacs is unable to have the window-start
point in the middle of a display property or overlay string.