[css-inline-3] Sizing layout bounds (line box) to fit fallback glyphs

[fantasai]

There are two independent ways in which the “size” of an inline box is relevant:

• what size is used for its content box, for painting its background
• what size is used when trying to figure out if it "fits" within the line box, or if the line box needs to grow (the “layout bounds”)

#5226 is about the first issue.

This issue is about the second one. Currently we only consider fallback glyphs when line-height is normal; in other cases we only consider the first available font. But we do consider glyphs in descendant boxes in all cases. So a few questions:

In particular, given the ability of text-edge to trim into the glyph which exacerbates any overflow/overwrite effects as a result of differing fallback font metrics, would we want to either make that possible, either as the default or as an option?

This is a follow-up issue from today's discussion on text-edge and leading-trim.

[dbaron]

Agree that this an #5226 are separate, although many of the same concerns and problems apply to both (such as most of #5226 (comment)).