1543482 - Hit-testing produces incorrect result with clip-path over iframe element

Status: RESOLVED FIXED

(Core :: Panning and Zooming, defect, P3)

Description

[Kartikaya Gupta (email:[e-mail])]

Attachment: testcase

STR:

• load attached file
• move mouse to over the blue area
• scroll with wheel/trackpad

Expected:

• iframe scrolls (blue rect will move)

Actual:

• root document scrolls, mostly. sometimes on first load it will scroll the iframe.

Comment 1

[Kartikaya Gupta (email:[e-mail])]

When the scrollframe for the iframe is inactive, we do a main-thread hit-test and it produces the right result. Once the scrollframe is activated, there is a hit-region in the layer tree for the bounding box of the clipped element, and so APZ always hits the wrong thing without waiting for the main thread.

As far as I can tell this has been broken since the original event regions code landed, which probably means that this scenario isn't exercised very much in the wild.

Comment 2

[Kartikaya Gupta (email:[e-mail])]

Also the testcase works correctly when WR is enabled, because we add the eIrregularArea flag for the clip-path element here which forces APZ to do main-thread lookup.

Comment 3

[Kartikaya Gupta (email:[e-mail])]

(In reply to Kartikaya Gupta (email:[e-mail]) from comment #2)

Also the testcase works correctly when WR is enabled, because we add the eIrregularArea flag for the clip-path element here which forces APZ to do main-thread lookup.

That's not true. The testcase does work correctly for WR, but it's because clip-path gets turned into an nsDisplayMasksAndClipPath item which goes directly into a WR clip item, and so WR can do precise hit-testing against it.

The eIrregularArea thing only happens for SVGs, so I should probably write a separate testcase for that.

Comment 4

[Kartikaya Gupta (email:[e-mail])]

(In reply to Kartikaya Gupta (email:[e-mail]) from comment #3)

That's not true. The testcase does work correctly for WR, but it's because clip-path gets turned into an nsDisplayMasksAndClipPath item which goes directly into a WR clip item, and so WR can do precise hit-testing against it.

Sadly, that's also not true. Only some nsDisplayMasksAndClipPath items (simple shapes) get precisely hit-tested. Anything that ends up as a mask clip looks like it gets turned into a bounding box here in WR and so we'll actually get incorrect results from WR in that case. The testcase I posted is a simple shape and so it works fine.

Comment 5

[Kartikaya Gupta (email:[e-mail])]

https://treeherder.mozilla.org/#/jobs?repo=try&group_state=expanded&revision=804f2844203bfe1187e3844b124f7adf66df9f8f

Comment 6

[Kartikaya Gupta (email:[e-mail])]

Attachment: Bug 1543482 - Extract a helper to identify clip paths WR can handle without masks. r?mattwoodrow

Comment 7

[Kartikaya Gupta (email:[e-mail])]

Attachment: Bug 1543482 - Add the irregular-area flag for frames with masks and clip paths. r?mattwoodrow,botond

This will force APZ to do a main-thread hit-test for the cases where the
shapes are too complex to easily send to the compositor.

Depends on D27454

Comment 8

[Kartikaya Gupta (email:[e-mail])]

(In reply to Kartikaya Gupta (email:[e-mail]) from comment #3)

The eIrregularArea thing only happens for SVGs, so I should probably write a separate testcase for that.

Filed bug 1544451 for that.

Comment 9

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/7d9507696389
Extract a helper to identify clip paths WR can handle without masks. r=mattwoodrow
https://hg.mozilla.org/integration/autoland/rev/7d6cd4e20705
Add the irregular-area flag for frames with masks and clip paths. r=mattwoodrow,botond

Comment 10

[Cristina Coroiu [:ccoroiu]]

https://hg.mozilla.org/mozilla-central/rev/7d9507696389
https://hg.mozilla.org/mozilla-central/rev/7d6cd4e20705

Comment 11

[Kartikaya Gupta (email:[e-mail])]

I don't think there's a pressing need to uplift this. It's a correctness improvement but not one we've run into in the wild.