1477121 - investigate doing reconstruction of a frame without reconstructing all of its children/descendants

Status: NEW

(Core :: Layout, enhancement, P3)

Description

[David Baron :dbaron: (⌚️UTC-4, no longer working on Mozilla)]

There are many cases where we reconstruct a frame, but we don't fundamentally need to reconstruct all of its descendants.  There are some invariants of the system that depend on this, and it's also probably a decent amount of work to figure out when this is safe, i.e., when changing some characteristic of a frame doesn't require changing some of its descendants (e.g., using different types of anonymous boxes, grouping them differently, etc.).  Though the latter issues may be similar to the set of cases where we move reconstruction a level up already (see, e.g., WipeContainingBlock).

It seems like this might be worth investigating since not destroying and rebuilding large subtrees of frames could save quite a bit of time, across frame construction, styling, and layout.