bug#42307: bug#42347: Feature request: Visual block attribute for overlays

[Gregory Heytings]

> > > > 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).
> > >
> > > What would be the definition of "whitespace" for this purpose?
> >
> > I think the regexp "[ \t\r\n]*" would be appropriate.
>
> Are you sure?
>
> First, \r doesn't happen in Emacs buffers,

I wasn't sure about \r, but included it just in case.

> and \n is the newline, so it isn't on the same line.

It is, but it's the last character of the line.  With the current default 
behavior it is displayed, there is one more blank character after each 
line, just type C-x = on that character.

In the two pictures (visualblock.png and visualblock-2.png) I sent (see 
bug #42307), the \n is removed on the right.  Otherwise there would be one 
more column on the right.

> Next, what about \f, and more generally about any other sequence of 
> characters that match [:blank:]?

Good question.  As far as I know \f is displayed by default as '^L' by 
Emacs, so it's a visible character, and I'm fine with this.  I think it 
should not count as a blank character, which means that an overlay with a 
'\f' in a column would extend at least to that column.

> And finally, what about stretches of whitespace generated by the 'space' 
> display properties?

I don't know, and I'm not sure I fully understand the question.  The 
"algorithm" I proposed is, I think, clear and simple: draw the overlay 
with :extend t, and trim whitespace on the left and on the right.

That being said, on a second thought I think a better name for the 
proposed attribute would be ":trim" (or perhaps ":trimleft" and 
"trimright" to give as much possible freedom to Emacs users).  It could 
have, for example, three values:

- default value = nil, gives the default behavior (with and without 
:extend)

- value = t, gives the behavior of "visiblechars.png"

- value = column, gives the behavior of "visualblock.png" and 
"visualblock-2.png"

Gregory