How would you handle dynamic row heights in a virtualized list?

Measure rows as they render, cache their heights, and maintain a running offset index (prefix sums) to map scroll position to row index. Use estimated heights for unmeasured rows and a ResizeObserver to catch changes after mount.

6 min read·~15 min to think through

Fixed-height virtualization is easy — index = scrollTop / rowHeight. Dynamic heights break that math, because you can't know a row's offset without knowing every prior row's height. This is the hard part of virtual scrolling.

The core problem

To render the visible window you need: given scrollTop, which rows are visible? With variable heights you need each row's cumulative offset, which depends on all earlier rows' actual heights — many of which haven't rendered yet.

Strategy

Estimate first. Give every unmeasured row an estimated height. The total scroll height = sum of estimates. The scrollbar is approximate but usable immediately.
Measure on render. When a row mounts, measure its real height (ref + getBoundingClientRect, or a ResizeObserver) and store it in a height cache keyed by row id/index.
Maintain a position index. Keep a prefix-sum array of offsets so "offset → index" and "index → offset" are fast lookups (binary search over the prefix sums = O(log n)). Update it as measurements come in.
Reconcile. When a measured height differs from the estimate, the total height and subsequent offsets shift. Update the spacer height; if the changed row is above the viewport, compensate scrollTop so the content doesn't visibly jump.
Watch for changes after mount — images loading, content expanding, font swaps. A ResizeObserver per rendered row catches these and re-measures.

Why it's tricky

Scroll anchoring — re-measuring a row above the viewport must not make the content jump under the user.
Estimate accuracy — bad estimates make the scrollbar lurch as the user scrolls into real content.
Measurement cost — measuring forces layout; do it in batches, not per-frame.
scroll-to-index for an unmeasured row is approximate until things settle.

Practical advice

This is exactly why you use a library — @tanstack/react-virtual and react-virtuoso implement measurement caches, ResizeObserver integration, and scroll anchoring properly. Building it yourself means owning all of the above.

Follow-up questions

•Why do you need a prefix-sum / position index?
•How do you prevent content from jumping when a row above the viewport is re-measured?
•How do you handle a row whose height changes after mount (image load)?
•How does scroll-to-index work when the target row hasn't been measured?

Common mistakes

•Assuming a single rowHeight and breaking on variable content.
•Not compensating scrollTop when re-measuring rows above the viewport.
•Measuring every row every frame, forcing constant layout.
•Poor height estimates causing the scrollbar to lurch.

Performance considerations

•Measurement forces synchronous layout — batch it and cache aggressively. Binary search over prefix sums keeps offset lookups O(log n). The estimate quality directly drives perceived scroll smoothness.

Edge cases

•Images or async content changing height after first render.
•Font swap (FOUT) shifting text height.
•Window resize re-flowing every row.
•Scroll-to-index into the far, unmeasured part of the list.

Real-world examples

•Chat apps where messages vary from one line to image attachments.
•Feeds with mixed media cards of unpredictable height.

Senior engineer discussion

Seniors identify the root difficulty — an offset depends on all prior heights — and reach for the estimate → measure → cache → prefix-sum loop, plus scroll anchoring to avoid jumps. They're explicit that this is the strongest argument for using a library rather than hand-rolling virtualization.