1713168 - Consider storing display item clipchains in the layout frame tree

Status: NEW

(Core :: Web Painting, enhancement, P3)

Description

[Miko Mynttinen]

Gecko stores paint time clipping information in a tree of DisplayItemClipChains. They are allocated during display list building and stored in display items. The clipchains and display items also store a pointer to ActiveScrolledRoot that describes what the clip moves with.

This approach has some issues:

• It produces duplicate clipchains (this problem is mitigated by deduplication, but memory allocations still happen)
• Gecko display items are bigger than necessary because they store a pointer to clipchains (twice because it might get mutated)
• Gecko clipchains are always rebuilt for new display items
• WR clipchains are always rebuilt when Gecko display list changes
• Clipchain lifetimes are unclear (clipchains are stored in a tree of reference counted pointers and released when the reference count reaches zero)

I think we could solve most of these issues by moving clipchains from display items to layout frame tree. This might not be easy and probably needs some planning.

Comment 1

[Jeff Muizelaar [:jrmuizel]]

(In reply to Miko Mynttinen from comment #0)

• Gecko display items are bigger than necessary because they store a pointer to clipchains (twice because it might get mutated)
  Twice only in WrapList items: https://searchfox.org/mozilla-central/rev/a9e776193401900a8ceffa478ae6e9930e895ca1/layout/painting/nsDisplayList.h#5013