GitHub details idle-gated height measurement and resize correction for rendered blocks

GitHub says it first gave every dynamic block a ResizeObserver that wrote measured heights back into the layout.

It rejected that design, saying an observer that writes a height into the layout it watches can retrigger itself, and the cost grows with every mounted block.

The replacement, GitHub says, runs a recurring measurement pass gated on idle time and scoped to about 2400px around the viewport. Mounted blocks serve as the ground truth, and observers normally only flag blocks for that pass rather than writing heights themselves. One exception applies a single synchronous correction before paint when a visible block resizes because of the user's action.