Adjust advance when justifying text

[kekelp]

Resolves #396.

[tomcur]

I think the direction makes sense, as the current state of LineMetrics::advance not taking padding into account certainly can be confusing. I've placed a few comments/questions inline.

[tomcur]

It's probably fine to store the unjustified advance here, as the user likely doesn't need to know.

[tomcur]

Could this be set (outside the loop) as line.metrics.advance = layout.alignment_width, for all except the final line? For very wide lines with many clusters, this iterative calculation will probably become quite lossy.

Does the code in the PR currently lead to a line.metrics.advance larger than the alignment_width, as trailing whitespace is counted twice? (I haven't run this locally yet.)

[kekelp]

Setting line.metrics.advance = layout.alignment_width, this is what happens:

The 2nd and 3rd lines use line.metrics.advance for the width of their selection box, so now it stops exactly at the end of the alignment_width. The first line calculates the selection box manually without using line.metrics.advance, and it shows that the trailing whitespace lives beyond layout.alignment_width, because the justification code ignores it.

We could just change the selection box code, of course, but the thing is that line.metrics.advance is documented as "Full advance of the line, including trailing whitespace". So I think it's correct that line.metrics.advance is larger than the alignment_width in this case.

Still, we can probably still avoid the iterative calculation by just setting line.metrics.advance = layout.alignment_width + line.metrics.trailing_whitespace. I'm not sure about the RTL line and the one with a trailing newline instead of a trailing space, but this seems to work as well: