1917340 - Firefox generates scrollable overflow (and creates unnecessary scrollbars) for transformed elements whose untransformed bounds would cause overflow

Status: NEW

(Core :: Layout: Scrolling and Overflow, defect, P3)

Description

[Daniel Holbert [:dholbert]]

STR:

1. Load https://bug1913927.bmoattachments.org/attachment.cgi?id=9423297 (testcase from bug 1913927)

ACTUAL RESULTS:
There's a vertical scrollbar on the black box

EXPECTED RESULTS:
No such vertical scrollbar.

We seem to be including the transformed element's untransformed-bounds in the scrollable overflow that we propagate up to the scroller. In some cases that's correct (e.g. when its untransformed bounds directly contribute to layout space), but in this case the parent element has a small fixed height, which apparently nerfs the scrollable-overflow-contribution from the transformed child, in other browsers.

Comment 1

[Daniel Holbert [:dholbert]]

This is not a regression (or not a recent one); I can reproduce the ACTUAL RESULTS at least as far back as Nightly 46.0a1 2016-01-01 with the linked testcase. (I didn't test any older builds than that.)

Chrome gives EXPECTED RESULTS and that seems to go back a ways, too; I tested Chrome 50 (also released in 2016) and I get expected-results there.

Comment 2

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

The severity field is not set for this bug.
:hiro, could you have a look please?

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