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From: Eli Zaretskii <eliz@gnu.org>
To: Vincent Lefevre <vincent@vinc17.net>
Cc: 44284@debbugs.gnu.org
Subject: bug#44284: 27.1; with some Unicode font, scrolling upward with the mouse wheel actually scrolls downward when the cursor needs repositioning
Date: Sun, 01 Nov 2020 20:47:04 +0200	[thread overview]
Message-ID: <834km8gbiv.fsf@gnu.org> (raw)
In-Reply-To: <20201101183458.GP27593@zira.vinc17.org> (message from Vincent Lefevre on Sun, 1 Nov 2020 19:34:58 +0100)

> Date: Sun, 1 Nov 2020 19:34:58 +0100
> From: Vincent Lefevre <vincent@vinc17.net>
> Cc: 44284@debbugs.gnu.org
> 
> > Thanks.  Do you have a suggestion for a fix?
> 
> Well, I think that there are 3 potential issues.
> 
> 1. That fact that here, extents.y_bearing is slightly incorrect,
> which appears to be a bug in Cairo.

Probably (I'm not an expert on Cairo), but if so, this is out of our
control.  For all I know, Cairo developers could have decided to drop
support for the font family you are using.

> 2. The fact that the cache->ascent integer gets incorrect in Emacs.
> 
> This could be fixed either on the Cairo side (see above) or on the
> application (Emacs) side. In the latter case:
> 
> First, if Emacs knows that under some condition, extents.y_bearing
> should be an integer, it could round to nearest. For instance, if
> this is an X font like here, is this necessarily the case?
> 
> Alternatively, Emacs could use some heuristic: if extents.y_bearing
> is very close to an integer, assume that it should be this integer.
> This is rather ugly as this might yield unexpected results in some
> applications, but would this be OK in Emacs?

It would be okay, I think: we have a few kludgey workarounds for
similar issues in other libraries.

I thought about something like using ceil (VALUE - 1/128) or maybe
even 1/256, on the assumption that the original integer value cannot
be of a greater granularity (the non-Cairo code uses 1/64).

> 3. The fact that row->phys_ascent > row->ascent in compute_line_metrics
> yields an incorrect behavior. This is about the following comment:
> 
>       /* If first line's physical ascent is larger than its logical
>          ascent, use the physical ascent, and make the row taller.
>          This makes accented characters fully visible.  */

This is not a bug, we must keep that code.  It just isn't supposed to
fire in your case.

> Or is the bug I've reported about that is specific to the incorrect
> cache->ascent issue (item 2), in which case fixing (2) would be
> sufficient for everyone?

Yes, we should solve item 2.

Thanks.





  reply	other threads:[~2020-11-01 18:47 UTC|newest]

Thread overview: 31+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-28 16:36 bug#44284: 27.1; with some Unicode font, scrolling upward with the mouse wheel actually scrolls downward when the cursor needs repositioning Vincent Lefevre
2020-10-28 16:44 ` Vincent Lefevre
2020-10-28 16:53 ` Eli Zaretskii
2020-10-30 10:52   ` Vincent Lefevre
2020-10-30 11:31     ` Eli Zaretskii
2020-10-30 13:33       ` Vincent Lefevre
2020-10-30 13:37         ` Eli Zaretskii
2020-10-30 16:31           ` Vincent Lefevre
2020-10-30 20:34             ` Vincent Lefevre
2020-10-30 20:57               ` Vincent Lefevre
2020-10-30 21:09                 ` Eli Zaretskii
2020-10-30 23:00                   ` Vincent Lefevre
2020-10-31  0:46                     ` Vincent Lefevre
2020-10-31  1:13                       ` Vincent Lefevre
2020-10-31  7:38                         ` Eli Zaretskii
2020-10-31 22:43                           ` Vincent Lefevre
2020-11-01  0:24                             ` Vincent Lefevre
2020-11-01  0:28                               ` Vincent Lefevre
2020-11-01 16:15                                 ` Eli Zaretskii
2020-11-01 16:15                               ` Eli Zaretskii
2020-11-01 17:32                                 ` Vincent Lefevre
2020-11-01 17:43                                   ` Eli Zaretskii
2020-11-01 18:34                                     ` Vincent Lefevre
2020-11-01 18:47                                       ` Eli Zaretskii [this message]
2020-11-01 21:13                                         ` Vincent Lefevre
2020-11-07  8:29                                         ` Eli Zaretskii
2020-11-07 10:35                                           ` Vincent Lefevre
2020-11-07 13:22                                             ` Eli Zaretskii
2020-11-01 15:55                             ` Eli Zaretskii
2020-11-01 17:26                               ` Vincent Lefevre
2020-10-30 20:58               ` Eli Zaretskii

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