1724471 - Transform on root element doesn't affect its background image

Status: NEW

(Core :: Web Painting, task, P3)

Description

[Daniel Holbert [:dholbert]]

STR:

1. Load attached testcase.

EXPECTED RESULTS:
The background image (in this case a gradient) should be rotated like the text is.

ACTUAL RESULTS:
The background image is not rotated.

I think this is responsible for these three WPT failures:
https://wpt.fyi/results/css/css-transforms/transform-root-bg-001.html
https://wpt.fyi/results/css/css-transforms/transform-root-bg-002.html
https://wpt.fyi/results/css/css-transforms/transform-root-bg-003.html

and it's also partly responsible for these failures:
https://wpt.fyi/results/css/css-transforms/transform-translate-background-001.html
https://wpt.fyi/results/css/css-transforms/transform-translate-background-002.html
(These two also have bug 1724459 to blame which I think is a distinct issue)

These tests are all included in the compat2021 test set here:
https://github.com/Ecosystem-Infra/wpt-results-analysis/blob/master/compat-2021/css-transforms-tests.txt

Comment 1

[Daniel Holbert [:dholbert]]

Attachment: testcase 1

Comment 2

[Emilio Cobos Álvarez [:emilio]]

This seems expected given background-image is propagated to the viewport, but transform isn't?

Comment 3

[Emilio Cobos Álvarez [:emilio]]

Ah, so this is this note in https://drafts.csswg.org/css-transforms/#transform-rendering:

Note: If the root element is transformed, the transformation applies to the entire canvas, including any background specified for the root element. Since the background painting area for the root element is the entire canvas, which is infinite, the transformation might cause parts of the background that were originally off-screen to appear. For example, if the root element’s background were repeating dots, and a transformation of scale(0.5) were specified on the root element, the dots would shrink to half their size, but there will be four times as many, so they still cover the whole viewport.

Seems pretty weird to me to "propagate" transform to the canvas as well, tbh.

Comment 4

[Daniel Holbert [:dholbert]]

(It sounds like comment 3 answered your question in comment 2? Canceling ni on that assumption.)

Comment 5

[Daniel Holbert [:dholbert]]

This same issue is also responsible for these WPT test-failures (also in the compat2021 test set):
https://wpt.fyi/results/css/css-transforms/transform-background-007.html
https://wpt.fyi/results/css/css-transforms/transform-background-008.html

Comment 6

[Daniel Holbert [:dholbert]]

Looks like there's an open spec issue on this (which is on the agenda for tomorrow's CSSWG meeting, though the agenda is packed so we may not get to it tomorrow): https://github.com/w3c/csswg-drafts/issues/6683