Axis-Split Sticky Constraints in Chromium Layout and Compositing

What this episode covers

Chromium is refining its rendering engine to support axis-split sticky constraints, allowing elements to stick to different scroll containers for horizontal and vertical movements. Previously, a single ancestor would trap both axes, but the new system enables independent layout and paint logic for each direction. Blink’s style system now distinguishes per-axis participation, while the layout phase identifies the nearest relevant scroller by skipping ancestors that do not scroll in a specific axis. In the pre-paint phase, the engine generates separate transform nodes for each axis, which are then passed to the compositor to manage independent clamping and offsets. This architectural shift ensures that complex nested scrolling—such as a header staying vertically fixed to the page while moving horizontally within a side-scrolling container—functions intuitively. Testing involves updated Web Platform Tests to validate that coordinate mapping and pixel snapping remain accurate across various writing modes and fragmented layouts.

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