1172767 - Scrolling an iframe inside a `preserve-3d` element causes a slow repaint

Status: RESOLVED WORKSFORME

(Core :: Layout, defect)

Description

[Christopher Van Wiemeersch [:cvan]]

Attachment: parent – scrolling test case

# Problem

Scrolling an iframe inside a `preserve-3d` parent makes the the iframe disappear for 5+ seconds while a repaint occurs (even though "preserve-3d is NOT inherited").


# Steps to reproduce

1. Create a parent document with an iframe placed inside an element that has a style set of `transform-style: preserve-3d`.
2. Load the page in fullscreen + VR mode.
3. Notice how long it takes for the iframe to load (even when the document is served locally).
4. Scroll the iframe (either by directly manipulating `document.document.scrollTop` or `window.scrollTo` on the iframe's `contentWindow` or via JS that gets called from within the iframe).
5. Notice it takes at least 5 seconds for the iframe's document to be updated (the content actually disappears, but the iframe container remains).

(I tried using the Dev Tools' profiler, but surprisingly nothing stood out.)


# Test cases

with an `<iframe>`: https://cvan.io/css-vr-slow-iframe-scrolling-tests/
with a `<div>`: https://cvan.io/css-vr-slow-iframe-scrolling-tests/div.html

Comment 1

[Christopher Van Wiemeersch [:cvan]]

Attachment: child – scrolling test case

Comment 2

[Christopher Van Wiemeersch [:cvan]]

Attachment: div – scrolling test case

Comment 3

[Vladimir Vukicevic [:vlad] [:vladv] (needinfo me, slow to respond)]

This seems painfully slow; we do disable a bunch of clipping and partial updates that we'd normally do while scrolling, but we should investigate what's actually being painted here.

Comment 4

[:kip (Kearwood Gilbert)]

Webvr is no longer dependent on this, but this is still affecting non-webvr sites.

Comment 5

[Gregory Pappas [:gregp]]

Runs very well now.