Fix hit-test z-offset computation for layers without CSS transforms in preserve-3d [chromium/src : main]
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Perry (Gerrit)
Mar 3, 2026, 9:01:39 AMMar 3
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Perry (Gerrit)
Mar 3, 2026, 9:58:20 AMMar 3
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Philip Rogers (Gerrit)
Mar 3, 2026, 7:53:45 PMMar 3
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Philip Rogers . unresolved
Can you put this behind an enabled-by-default runtime enabled feature?
Can you help me understand why only 2 of the 3 calls to IsHitCandiateForDepthOrder are updated?
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Perry (Gerrit)
Mar 3, 2026, 10:29:07 PMMar 3
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Philip Rogers . resolved
Can you put this behind an enabled-by-default runtime enabled feature?
Can you help me understand why only 2 of the 3 calls to IsHitCandiateForDepthOrder are updated?
Perry
The three calls serve two distinct purposes.
1. The two updated calls compute the z-offset of the current layer itself. `local_transform_state` was wrong here because `CreateLocalTransformState` either calls `Flatten()` (resetting z to 0) or adds a 2D paint offset
(introducing floating-point rounding differences). Since a layer's z in a preserve-3d context is determined solely by its transform container's accumulated transform, `container_transform_state` is correct.
2. The third call (in `HitTestChildren`) compares a child layer's z (already written into `*z_offset` by the child's own recursion) against the calling layer's z. This call doesn't need updating because
`local_transform_state` and `container_transform_state` always yield the same `ComputeZOffset` result here: either the calling layer has a CSS transform and `local_transform_state == container_transform_state` by
definition (line 1373), or it has no CSS transform and the two states differ only by a 2D paint offset which doesn't affect z.
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Perry (Gerrit)
Mar 4, 2026, 1:09:14 AMMar 4
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File-level comment, Patchset 5 (Latest):
Perry . resolved
Runtime feature is added, please take a look again, thanks.
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Perry (Gerrit)
Mar 5, 2026, 9:18:03 PMMar 5
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Philip Rogers (Gerrit)
Mar 6, 2026, 9:06:54 PMMar 6
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Perry (Gerrit)
Mar 6, 2026, 9:08:03 PMMar 6
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Chromium LUCI CQ (Gerrit)
Mar 6, 2026, 10:03:47 PMMar 6
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Change information
Commit message:
Fix hit-test z-offset computation for layers without CSS transforms in preserve-3d
When computing z-offset for depth ordering in a preserve-3d context,
IsHitCandidateForDepthOrder was called with local_transform_state, which
is derived from container_transform_state by applying the layer's local
2D paint offset. This caused two problems:
1. For layers that are not direct children of the transform container
element, CreateLocalTransformState calls Flatten(), which resets
accumulated_transform_ to identity, causing ComputeZOffset to return
0 instead of the correct z value.
2. For direct children, the extra 2D translate operations added to
local_transform_state introduce floating-point rounding differences
compared to sibling layers that went through Flatten(), causing
incorrect depth ordering between coplanar descendants.
The correct transform state for computing a layer's z-depth relative to
its containing flattening layer is container_transform_state itself,
since layers without their own CSS transforms are coplanar with the
transform container and their z-depth is fully determined by the
container's accumulated transform.
Bug: 464173566
Change-Id: Ib07c40e4239bacc6ff741408ca7d9fcf1332f91b
Reviewed-by: Philip Rogers <p...@chromium.org>
Commit-Queue: Perry <perry...@gmail.com>
Cr-Commit-Position: refs/heads/main@{#1595858}
Files:
- M third_party/blink/renderer/core/paint/paint_layer.cc
- M third_party/blink/renderer/platform/runtime_enabled_features.json5
- A third_party/blink/web_tests/external/wpt/css/css-transforms/hittest-preserve-3d-2.html
Change size: M
Delta: 3 files changed, 137 insertions(+), 4 deletions(-)
Branch: refs/heads/main
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Blink W3C Test Autoroller (Gerrit)
Mar 7, 2026, 12:35:55 AMMar 7
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The WPT PR for this CL has been merged upstream! https://github.com/web-platform-tests/wpt/pull/58344
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