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bug#66942: move-to-column behavior when newlines have a display property


From: JD Smith
Subject: bug#66942: move-to-column behavior when newlines have a display property
Date: Sun, 5 Nov 2023 12:18:12 -0500

Eli Zaretskii <eliz@gnu.org> writes:

> ...unless someone comes with an idea for how to handle these situations in a 
> sensible way (and I thought long and hard about it, but couldn't find such 
> ideas), this will remain a limitation of move-to-column, and one of the 
> complications introduced by display strings with embedded newlines in general.

Would it not make more sense to place the cursor _before_ the first character 
with a `\n` in its replacing-display property, if move-to-column attempts to 
move past it?  It is true that neither that position nor a position on a 
subsequent line is “correct”, but the former at least preserves the expected 
move-to-column semantics of “stay on this line”.  

The final newline is the only place to add display on blank lines, so this will 
be a general problem for any package attempting to do that.  Here’s a simple 
test of this for people wanting to investigate:

(progn
  (insert "\n>>>>\n" (propertize "\n" 'display "xxxxx\n") "SHOULD NOT MOVE 
HERE\n<<<<")
  (forward-line -2) ; at beginning of xxxxx\n line
  (move-to-column 10)) ; should not move past the line




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