951467 - make scroll layer items report the correct bounds and visible region

Status: RESOLVED FIXED

(Core :: Layout, defect)

Description

[Timothy Nikkel (:tnikkel)]

Currently the bounds of scroll layer items are just the union of the bounds of child items (inherited from nsDisplayWrapList). This has two problems. If a displayport is set the bounds will be much larger than what is drawn to the screen (only part of the displayport will be composited and clipped into the scroll port). Also the displayport can legally be set to be something that does not intersect the scroll port at all (if we are panning quickly), making the (clipped) bounds empty. This was (one of) the issues in bug 936500. We fixed it by making scroll layer items always visible even if they have empty visible rect.

Instead what we really need is a concept of 'inside' bounds/visible region and 'outside' bounds visible region, with scroll layer items handling the any translation needed and giving the correct bounds wherever they are needed. The outside bounds of scroll layer items are just the scroll port, the inside bounds are the display port. This is similar to pre-rendered transform items that have an expanding visible region for things that aren't (yet) visible.

Comment 1

[Timothy Nikkel (:tnikkel)]

Attachment: patch

Comment 2

[Timothy Nikkel (:tnikkel)]

https://hg.mozilla.org/integration/mozilla-inbound/rev/21837cd4fcb1

Comment 3

[Carsten Book [:Tomcat]]

https://hg.mozilla.org/mozilla-central/rev/21837cd4fcb1

Comment 4

[Stephen A Pohl [:spohl] (OOO until 3/25)]

Thank you! This fixes at least bug 932055 on Metro, and there are other bugs that may turn out to be duplicates as well.

Comment 6

[Stephen A Pohl [:spohl] (OOO until 3/25)]

This should track 28 since bug 932055 was tracking 28.

Comment 8

[Lukas Blakk [:lsblakk] use ?needinfo]

Let's get an uplift nomination on this.

Comment 9

[Timothy Nikkel (:tnikkel)]

Comment on attachment 8349125 [details] [diff] [review]
patch

[Approval Request Comment]
Bug caused by (feature/regressing bug #): not sure, the code has been this way for a while, it might have just been exposed more recently due to other changes.
User impact if declined: fixes at least bug 932055 and bug 941003
Testing completed (on m-c, etc.): on m-c for a few weeks now
Risk to taking this patch (and alternatives if risky): changes should be well contained, regressions should show up relatively quickly
String or IDL/UUID changes made by this patch: none

Comment 10

[Ryan VanderMeulen [:RyanVM]]

https://hg.mozilla.org/releases/mozilla-aurora/rev/08706d24334b

Comment 11

[u279076]

Does this have or need tests?

Comment 12

[Timothy Nikkel (:tnikkel)]

It could use some sure, but it's not immediately clear to me how to write one as I never reproduced bug 932055 or bug 941003.