The parent says 10px, the child says inherit, and zoom quietly turns that into the wrong number.
Status: 🚧 In Review
The Unexpected Interaction
CSS zoom changes the effective scale used when lengths become computed values. Explicit inherit copies a computed value from the parent. Put the two together and Blink has to preserve the author's value while moving it between elements with different effective zoom.
That sounds abstract, but the failure is concrete: a parent computes a length under one zoom, a child with another zoom says inherit, and the inherited value is scaled twice or not re-scaled when it should be. The same shared bug appeared across otherwise unrelated length properties.
Why Property-by-Property Fixes Were Wrong
Early failures looked local: one property serialized incorrectly, another had a bad WPT expectation, SVG text decoration had a special override. Fixing each symptom would leave the inheritance machinery inconsistent.
The general fix marks properties whose computed values are affected by zoom and recomputes explicitly inherited values for the child's effective zoom. That lets the style system handle length properties as one class of problem rather than a collection of exceptions.
- MERGED Fix remaining zoom inheritance failures
- IN REVIEW Fix explicit inheritance for all length properties
A Tree-Scope Trap in animation-timeline
animation-timeline exposed a second layer. Its computed-value serialization loses the tree scope attached to the original declaration. Reapplying that serialized value during explicit inheritance reached CSSToStyleMap::MapAnimationTimeline() without a scoped value and hit a DCHECK.
The fix re-scopes the serialized value to the child element's tree scope and marks the property as zoom-sensitive so pixel insets in view() timelines are recomputed correctly.
Removing an Old SVG Exception
SVG <text> still carried a StyleAdjuster override from the SVG Text NG transition that forced text-decoration-thickness to auto. Removing it let explicit thickness work again, and marking the property zoom-sensitive made inheritance use the child's effective zoom.
Testing the Rule, Not the Examples
A computed-value harness now exercises explicit inherit across zoom levels. The important part is coverage at the abstraction boundary: any zoom-sensitive length property should obey the same invariant, including future properties not named in the original bug.
The Takeaway
When unrelated CSS properties fail the same computed-value pattern, the property implementations are probably not the right layer to patch. The better fix is to identify the shared conversion - here, explicit inheritance across effective zoom - and teach the style system the invariant once.