[csswg-drafts] [css-inline-3] Sizing layout bounds (line box) to fit fallback glyphs (#5238)

fantasai has just created a new issue for https://github.com/w3c/csswg-drafts:

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

https://github.com/w3c/csswg-drafts/issues/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](https://lists.w3.org/Archives/Public/www-style/2020Jun/0019.html).

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/5238 using your GitHub account