[WC-3390] DG2: improve virtual scroll behavior and height logic

[yordan-st]

Pull request type

Bug fix (non-breaking change which fixes an issue)

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Description

• Virtual scrolling got stuck showing only the initial page of rows when the grid had many columns. The overflow detection compared measurements from different sources, incorrectly skipping the offset needed to make the body scrollable. Fixed by using consistent measurements for the overflow check and adding a fallback that loads more rows when the grid has no scrollbar but more data is available.

[github-actions]

AI Code Review

⚠️ Approved with suggestions — low-severity items only, safe to merge

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What was reviewed

File	Change
packages/pluggableWidgets/datagrid-web/src/model/stores/GridSize.store.ts	Added fallback in lockGridContainerHeight to call bumpPage() when no scrollbar exists but more data is available
packages/pluggableWidgets/datagrid-web/CHANGELOG.md	Added ### Fixed entry under [Unreleased]
packages/pluggableWidgets/datagrid-web/e2e/filtering/DataGridFilteringIntegration.spec.js-snapshots/datagridFilteringIntegration-chromium-linux.png	Updated screenshot baseline

Skipped (out of scope): dist/, pnpm-lock.yaml

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Findings

⚠️ Low — No unit tests for the new fallback path

File: packages/pluggableWidgets/datagrid-web/src/model/stores/GridSize.store.ts lines 116–119
Note: The new bumpPage() fallback (called when scrollHeight <= clientHeight but hasMoreItems is true) has no corresponding unit test. The store is otherwise untested. The scenario — grid body does not overflow but more items exist — is precisely the regression risk described in the PR. A unit test that mocks gridBodyRef.current with matching scrollHeight/clientHeight values and asserts setPageAction is called would lock this behaviour in.

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⚠️ Low — lockGridContainerHeight re-checks hasMoreItems redundantly

File: packages/pluggableWidgets/datagrid-web/src/model/stores/GridSize.store.ts line 117
Note: The method already returns early at line 79 when !this.hasMoreItems, so the guard this.hasMoreItems && on line 117 is always true by the time execution reaches it. It is harmless but adds noise; the condition can be simplified to just gridBody && gridBody.scrollHeight <= gridBody.clientHeight.

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Positives

• The CHANGELOG entry under [Unreleased] is well-written: end-user language, describes both the symptom ("showing only the initial page of rows") and the trigger condition ("many columns") without exposing implementation details.
• The fix is surgical — only 5 lines added, no unrelated changes pulled in, and the existing early-exit guard at line 79 ensures the new code only runs during virtual scrolling with pending data.
• Updated the Playwright screenshot baseline alongside the fix, preventing a stale-baseline CI failure.

[r0b1n]

It feels this has to be consolidated with #2167 and addressed together, while those might be different issues, they are caused by the came code which needs an overhaul/thinking though session.