bug#72721: 31.0.50; Visual-wrap-prefix-mode breaks Magit log buffers

[Jim Porter <jporterbugs@gmail.com>]

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On 8/19/2024 2:39 PM, Gautier Ponsinet wrote:
> Hello everyone,
> 
> The new visual-wrap-prefix-mode breaks the rendering of the Magit Log
> buffers.
> 
> In emacs -Q:
> * Install Magit and its dependencies and load Magit.
> * Go to a local repository (via M-x dired or M-x cd).
> * M-x global-visual-wrap-prefix-mode
> * M-x magit-log-current
> 
> Could someone please confirm/reproduce?

I can confirm this. I'm not quite sure of all the details, but it seem 
that this is due to a bad interaction between overlays and the 
'min-width' display spec. The end result was that we were calling 
'get-text-property' with a (large-ish) buffer position when the OBJECT 
arg was a string of length 1. That can happen in magit-log on the 
mostly-blank line where it's making the ASCII art just below a merge 
commit. (The leading whitespace makes 'visual-wrap-prefix-mode' do its 
thing.)

I'm not super familiar with how the display engine works, but I think we 
don't want to call 'display_min_width' when we're working with an 
overlay. See the attached patch.

Eli, I'm sure you understand this code much better than me. Does the 
above make sense? I can also try to improve the commentary in the code, 
but I'm just making some educated guesses as to what's happening here.

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From da02157ee69c6ae2a220a8f38a9cd8c0f7105641 Mon Sep 17 00:00:00 2001
From: Jim Porter <jporterbugs@gmail.com>
Date: Mon, 19 Aug 2024 17:38:47 -0700
Subject: [PATCH] Fix bad interaction between 'min-width' display spec and
 overlays

* src/xdisp.c (handle_display_prop): Don't handle 'min-width' when
iterating over a string (bug#72721).
---
 src/xdisp.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/src/xdisp.c b/src/xdisp.c
index 30771a1c83d..bd6657fe522 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -5791,14 +5791,18 @@ handle_display_prop (struct it *it)
   propval = get_char_property_and_overlay (make_fixnum (position->charpos),
 					   Qdisplay, object, &overlay);
 
-  /* Rest of the code must have OBJECT be either a string or a buffer.  */
+
   if (!STRINGP (it->string))
-    object = it->w->contents;
+    {
+      /* Rest of the code must have OBJECT be either a string or a buffer.  */
+      object = it->w->contents;
 
-  /* Handle min-width ends. */
-  if (!NILP (it->min_width_property)
-      && NILP (find_display_property (propval, Qmin_width)))
-    display_min_width (it, bufpos, object, Qnil);
+      /* Handle min-width ends.  This doesn't apply while iterating over
+	 a string.  */
+      if (!NILP (it->min_width_property)
+	  && NILP (find_display_property (propval, Qmin_width)))
+	display_min_width (it, bufpos, object, Qnil);
+    }
 
   if (NILP (propval))
     return HANDLED_NORMALLY;
-- 
2.25.1