bug#40507: 26.2; truncate-lines does not respect line-height

[Eli Zaretskii]

severity 40507 wishlist
thanks

> From: ndame <emacsuser@freemail.hu>
> Date: Wed,  8 Apr 2020 15:51:09 +0200 (CEST)
> 
> Even if the newline is not on screen, it should affect the height of the 
> truncated 
> line, because that's why the package developer added line-height to the 
> newline,
> so the  line has some spacing.

That's not what this feature was designed for.  It is conceptually the
same as having a character in a line whose height is greater than
those of others: the line's hight will be increased only if that
taller character is displayed.

The line-height property you put on a newline is basically a means to
force the newline character have some specified height, even though
the newline doesn't have a glyph with a definite metrics.  That's why
this property is placed on a newline, not on any other character.

In terms of the implementation, the feature you expected would require
the display engine consider characters that are not visible in the
window, something that the display code tries hard to avoid doing (so
it could be faster).