[css-inline-3] Rethinking line-sizing and leading-trim #5168

I wanted to follow up on some thoughts:

• dbaron's musings in A Coruña
• Issues around line-sizing: normal’s model working fine if line-height has significant slack, but not if it doesn't: the half-leading on the root element isn't enough to handle slight shifts in ascent/descent due to varying font-families, for example, if there's hardly any half-leading. See this video from ~11:37 - 12:30 (or just watch the whole thing if you prefer ;)
• Handling descendant inlines on the first/last line for leading-trim
• Possible benefits of splitting the choice of metric into an inheritable, independently-cascading property, leaving leading-trim to focus on whether or not to trim.

Starting from some of dbaron's comments, I suspect reworking the relationship of these two features might solve a lot of these issues.

For example, what if instead of line-sizing: legacy | normal (inheritable) and leading-trim-over/under: normal | <metric> (non-inheritable, applies to block containers, inline boxes) we had:

• text-edge-over/under: leading | normal | <metric> (inheritable, applies to inline boxes except root inline)
  • leading inline box does the usual thing of contributing half-leading
  • normal inline box ignores 'line-height', contributes margin-box edge
  • <metric> is like normal, but also trims the content-height from ascent/descent down to specified metric
• leading-trim: normal | start | end | both (non-inheritable, applies to block containers)
  • normal applies half-leading over/under the root inline box on all lines
  • start trims outer half-leading of root inline box on first line, down to text-edge metric
  • end trims outer half-leading of root inline box on last line, down to text-edge metric