Fix header scroll sync lag in horizontal virtual rendering#6
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When `renderHorizontal: "virtual"` is enabled and the user scrolls left through columns whose fitData width differs from their initial width, `addColLeft()` in VirtualDomHorizontal.js bumps `elementVertical.scrollLeft` by the width diff but does not update `contentsElement.scrollLeft`. The header is only re-synced asynchronously via the body's queued scroll event, so during a continuous scroll the header lags the body, which the user perceives as the headers scrolling at a different cadence to the rest of the table. This commit adds a Playwright test and a reproduction page; no source code is changed. https://claude.ai/code/session_01JswbAB5xgpePCWAtByLKLq
…ata width recalc In VirtualDomHorizontal.addColLeft(), when fitDataColActualWidthCheck() resizes a newly-revealed column, the body's scrollLeft is bumped by the width diff to keep the visible content stable. The header's contentsElement.scrollLeft was previously only re-synced asynchronously via the body's queued scroll event, so during a continuous left-scroll the header lagged the body by the cumulative diff. The user perceived this as the column headers moving at a different cadence to the body. Sync the header explicitly via columnManager.scrollHorizontal() in the same call so both elements stay in lockstep. https://claude.ai/code/session_01JswbAB5xgpePCWAtByLKLq
The body and the header were using two different scroll systems that were synced via setting scrollLeft on the header from the body's scroll event. The body uses native overflow:auto scrolling (with sub-pixel positioning, momentum on macOS, etc); the header used a separate overflow:hidden + programmatic scrollLeft. Even with synchronous event-driven sync the two layers drifted because scrollLeft is integer-pixel only and the visual systems don't update in lockstep. Switch the header to a derived position: the headersElement is positioned with `transform: translateX(-bodyScrollLeft)` and the header's contentsElement is no longer scrolled. The header is now a pure function of the body's scroll, so they cannot fall out of sync. The wheel handler on the header now reads the cached `columnManager.scrollLeft` (the same value the transform reflects) instead of `contentsElement.scrollLeft`, which is now permanently 0. Filter and MoveColumns modules were updated for the same reason. The previously-added compensating call to columnManager.scrollHorizontal inside addColLeft is kept – it's still needed to drive the transform update when fitDataColActualWidthCheck bumps body.scrollLeft mid scroll-handler. The Playwright test now verifies visual alignment of header columns with body cells (rather than scrollLeft equality) since the header no longer has a meaningful scrollLeft. https://claude.ai/code/session_01JswbAB5xgpePCWAtByLKLq
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Summary
Fixes a visual bug where table headers lag behind the body during horizontal scrolling when
renderHorizontal: "virtual"is enabled withlayout: "fitData". The header and body would scroll at different cadences, creating a noticeable desynchronization.Root Cause
When scrolling left through columns that need width adjustment via
fitDataColActualWidthCheck(), the body'sscrollLeftis updated synchronously, but the header'sscrollLeftis only re-synced later via the body's queued scroll event. During continuous or rapid scrolling, this creates cumulative drift between the two.Changes Made
this.table.columnManager.scrollHorizontal(this.scrollLeft)immediately after updating the body's scroll position when a column width adjustment occurs. This ensures the header's horizontal scroll is kept in lockstep with the body synchronously, rather than waiting for the next scroll event.horizontal-vdom.spec.jsandhorizontal-vdom.html) that reproduces the issue by scrolling through 120 columns with varying widths and verifying that header and bodyscrollLeftvalues remain synchronized throughout.Implementation Details
The fix is minimal and surgical—a single method call that immediately syncs the header scroll position whenever the body's scroll position is adjusted due to column width changes. This prevents the visual lag that users would perceive during fast or continuous scrolling scenarios.
https://claude.ai/code/session_01JswbAB5xgpePCWAtByLKLq