bug#42307: Feature request: Visual block attribute for overlays

[Gregory Heytings via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

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In Emacs 21 to 26, overlays between two points in the buffer on two 
different lines extend to the right border of the window.  This has 
changed in Emacs 27, and now the default is that overlays extend only one 
character position after the last character of the line.  The previous 
behavior can be obtained with the ":extend t" face attribute. I agree that 
the earlier behavior was not optimal, but I think the current behavior 
with its staircase aspect is (at least for some cases) not optimal either.

I think a third way to display overlays would make sense, and would be 
better than the earlier and current defaults.  Let's name this attribute 
":visualblock".  It would produce the following result:

1. calculate the overlay as it would have been displayed by Emacs 21-26.

2. remove all pixel columns on the right *and on the left* of the overlay 
which have no "content" (that is, no characters on the right, and 
whitespace characters on the left).

With this, on overlay on, for example, a block of code between () or {} 
would be displayed on the screen as a block.  The above "algorithm" works 
with fixed and variable-width fonts, but could be made more efficient for 
fixed-width fonts.

To give an example of what I mean, I attach three pictures, taken with 
(show-paren-mode) and (setq show-paren-style 'expression), showing 
respectively the earlier default, the current default, and the proposed 
"visualblock" feature.

Gregory

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