1697311 - mask-composite doesn't work properly without mask-repeat: no-repeat or a border-radius > padding (or border-width)

Status: RESOLVED FIXED

(Core :: Web Painting, defect)

Description

[Ana Tudor]

Attachment: firefox_result_buggy.png

User Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/86.0.4240.75 Safari/537.36

Steps to reproduce:

Trying to get a gradient frame using masking.

1. Set a non-zero padding or (transparent border) on the element with the gradient.

padding: 1em;
width: 8em; height: 8em;
background: linear-gradient(gold, indigo) border-box;

2. Set a mask consisting of two fully opaque layers, one limited to content-box by mask-clip, the other one covering the entire border-box.

--grad: linear-gradient(red, red);
mask: 
  var(--grad) content-box, 
  var(--grad)

3. Set mask-composite: exclude - this should basically XOR the two layers, leaving just the frame created by the padding (or border) visible.

Actual results:

Everything disappears when setting mask-composite: exclude unless:

a) we also set mask-repeat: no-repeat

OR

b) we also set a border-radius value that's greater than that of the padding (or border-width) - if the border-radius is smaller or equal to the padding, this doesn't work

Reduced test case https://codepen.io/thebabydino/pen/ZEBmLQQ

Expected results:

We should get a gradient frame. This is what happens in Chrome.

Comment 1

[BugBot [:suhaib / :marco/ :calixte]]

The Bugbug bot thinks this bug should belong to the 'Core::Layout' component, and is moving the bug to that component. Please revert this change in case you think the bot is wrong.

Comment 2

[Emilio Cobos Álvarez [:emilio]]

Attachment: Testcase from reporter.

Comment 3

[Emilio Cobos Álvarez [:emilio]]

This worked briefly (sometime between firefox 55 and 58). Regression range: https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=307a7a34013060a6a1e87dfbb911f058d0781a2e&tochange=57f68296c350469d73d788eb3695a898947b4acb

There's a lot of stuff there because we don't have all those intermediate builds, but from that list bug 1373750 looks particularly suspicious.

Comment 4

[Emilio Cobos Álvarez [:emilio]]

Attachment: Bug 1697311 - Simplify GetInsideClipregion(). r=mstange

The code was unconditionally passing the border area, which is very
suspect, and causes us to repeat the mask incorrectly towards the
border, drawing nothing.

Comment 5

[Emilio Cobos Álvarez [:emilio]]

Attachment: Bug 1697311 - Add a comment and clean up nsDisplayMasksAndClipPaths constructor. r=mstange

The code is unconditionally passing the border area, which is very
suspect, but it's not problematic because it only cares of getting the
dest rect (which doesn't depend on the background clip).

Depends on D107747

Comment 6

[Emilio Cobos Álvarez [:emilio]]

Attachment: Bug 1697311 - Use the right clip for subsequent mask layers. r=mstange

The code was correctly computing currentLayerClipState for the right
layer, but then it was calling PrepareImage with clipState rather than
currentLayerClipState, which in the case of masks only contains the
clip for the bottom layer. This can cause background-repeat to
over-extend, causing the issue.

Depends on D107757

Comment 7

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/47e251fa2c9f
Simplify GetInsideClipregion(). r=mstange
https://hg.mozilla.org/integration/autoland/rev/2f69ef9296a3
Add a comment and clean up nsDisplayMasksAndClipPaths constructor. r=mstange
https://hg.mozilla.org/integration/autoland/rev/350441a59119
Use the right clip for subsequent mask layers. r=mstange

Comment 8

[Web Platform Test Sync Bot [:wpt-sync] (Matrix: #interop:mozilla.org)]

Created web-platform-tests PR https://github.com/web-platform-tests/wpt/pull/28006 for changes under testing/web-platform/tests

Comment 9

[Dorel Luca [:dluca]]

https://hg.mozilla.org/mozilla-central/rev/47e251fa2c9f
https://hg.mozilla.org/mozilla-central/rev/2f69ef9296a3
https://hg.mozilla.org/mozilla-central/rev/350441a59119

Comment 10

[Web Platform Test Sync Bot [:wpt-sync] (Matrix: #interop:mozilla.org)]

Upstream PR merged by moz-wptsync-bot