Fix LayoutShift attribution data to use CSS pixels #55329
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The LayoutShift API was returning attribution data (currentRect and prevRect) in physical pixels instead of CSS pixels. This makes the Layout Instability API inconsistent with other layout-related APIs, which all use CSS pixels. With the current behavior, consumers of this API are required to manually convert the attribution data using devicePixelRatio to make the coordinates relative to the viewport. Previously, on a display with device pixel ratio 2.0, a 100x100 CSS pixel element positioned at (50, 50) would be reported as 200x200 at (100, 100) in the attribution data. After this fix, it correctly reports as 100x100 at (50, 50) CSS pixels, matching the actual element position and size as seen by web developers. The layout shift scores themselves remain unaffected as they are calculated using the impact fraction (shifted area relative to viewport) multiplied by the distance fraction (movement distance relative to viewport dimensions). Only the attribution data reporting to the Performance API is corrected. Bug: 399058544 Change-Id: Ib8109c12eaf1c1a2e5648edbc39ef97111431494 Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6624567 Reviewed-by: Manuel Rego <[email protected]> Reviewed-by: Michal Mocny <[email protected]> Commit-Queue: Manuel Rego <[email protected]> Cr-Commit-Position: refs/heads/main@{#1527545}
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The LayoutShift API was returning attribution data (currentRect and
prevRect) in physical pixels instead of CSS pixels. This makes the Layout
Instability API inconsistent with other layout-related APIs, which all use
CSS pixels.
With the current behavior, consumers of this API are required to manually
convert the attribution data using devicePixelRatio to make the coordinates
relative to the viewport.
Previously, on a display with device pixel ratio 2.0, a 100x100 CSS pixel
element positioned at (50, 50) would be reported as 200x200 at (100, 100)
in the attribution data. After this fix, it correctly reports as 100x100
at (50, 50) CSS pixels, matching the actual element position and size as
seen by web developers.
The layout shift scores themselves remain unaffected as they are calculated
using the impact fraction (shifted area relative to viewport) multiplied
by the distance fraction (movement distance relative to viewport dimensions).
Only the attribution data reporting to the Performance API is corrected.
Bug: 399058544
Change-Id: Ib8109c12eaf1c1a2e5648edbc39ef97111431494
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6624567
Reviewed-by: Manuel Rego <[email protected]>
Reviewed-by: Michal Mocny <[email protected]>
Commit-Queue: Manuel Rego <[email protected]>
Cr-Commit-Position: refs/heads/main@{#1527545}