A code row is a line at a known height. You can compute the whole geometry up front and never have to correct it. Comments don’t have a predictable height. So we stopped forcing one geometry to serve both, and split the document into two independent height domains.

For the first design, we gave every dynamic block a ResizeObserver that wrote measured heights back into the layout. We built it, then rejected it. An observer that writes a height back into the layout it’s watching can retrigger itself, and the cost grows with every mounted block.

What we shipped is a recurring measurement pass, gated on idle and scoped to roughly 2400px around the viewport. Mounted blocks are ground truth. Observers normally only flag a block for the pass. They don’t write heights themselves. There’s one exception: a visible block resizing because of the user’s action can apply one synchronous correction before paint. That keeps the comment and code below it moving together without reopening the feedback loop.