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bug#8627: 24.0.50: cursor property behaves irregularly in before-strings

From: Alp Aker
Subject: bug#8627: 24.0.50: cursor property behaves irregularly in before-strings
Date: Fri, 06 May 2011 17:17:16 -0400 (EDT)

But if the before-string contains a newline, the cursor property
appears to be ignored.

I will take a look at fixing this, but can you show a real-life use
case where this is needed?

I ran into the need while working on this package:


It shades the area past the fill column by putting overlays on newlines (if you look at the screenshot it'll be obvious how it works). In a few edge cases it would be useful to be able to display a newline that's not part of the buffer content. For example, if the last line of the buffer doesn't end in a newline, then no fill-column shading appears on that line (since there's no newline to propertize). I'd like to put an overlay at point-max with an after-string containing a propertized newline character, so that the fill-column shading can appear on the last line of the buffer. But I can't do so, because the newline in the after-string causes any cursor property in it to be ignored, and so cursor motion at the end of the buffer becomes deranged. (Display strings won't do here, because I don't want to replace the display of any characters in the buffer.)

Does that qualify as a use case?

I might as well also mention, at least for the record, another (minor) issue with cursor properties in before-strings that I came across: If the position with the cursor property is out of sight because of horizontal scrolling, then the property is ignored, even when auto-hscroll-mode is enabled. Perhaps it would make more sense to respect the cursor property when auto-hscroll-mode is non-nil, and adjust hscrolling to bring the requested cursor position into view?

Here's a test case:

  (setq auto-hscroll-mode t)
  ;; A line of text, moving to the end of which will trigger hscrolling.
  (insert (make-string (+ hscroll-margin (window-width) 5) ?X))
  (setq m (point-marker))
  (insert "\n\n")
  (setq o (make-overlay (1- (point)) (point)))
  ;; Request that cursor be at bol, but make the before-string long
  ;; enough that if the cursor property is ignored we won't return from
  ;; hscrolling.
  (overlay-put o
                (propertize " " 'cursor 1)
                (make-string (window-width) 32)))
  (goto-char m)
  (forward-line 1))

After evaluation the cursor will be at the end of the before-string, rather than at the requested position. (This happens with 22 and 23 as well.)

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