Fix PaginationState UI not refreshing on filtered count reduction (#58487)

[Pinten84]

Description

Fixes #58487

Addresses a race condition in PaginationState where TotalItemCountChanged was not raised when the item count reduction triggered a page index reset.

Bug Logic

1. User is on Page 10 of 10 (100 items).
2. Filter reduces items to 5.
3. SetTotalItemCountAsync(5) detects invalid page -> calls SetCurrentPageIndexAsync(0).
4. SetCurrentPageIndexAsync triggers a data refresh, calling SetTotalItemCountAsync(5) again.
5. The second call sees TotalItemCount is already 5, so it exits early without raising the event.
6. Result: UI stays on "Page 10" (phantom state).

Fix

Added a _hasPendingTotalItemCountChangedEvent flag.

• When a page reset is required, we set the flag.
• The subsequent recursive call checks this flag and raises the event even if the count hasn't changed.

Acknowledgements

Special thanks to @htmlsplash for identifying the root cause and proposing the solution in the issue discussion.

Verification

Added PaginationTests suite with regression test for the bug where TotalItemCountChanged fails to fire after page reset when the subsequent re-query returns the same total count. Additional tests cover page reset behavior, LastPageIndex calculation, event firing, and page preservation scenarios.

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[Copilot]

Pull request overview

This PR fixes a race condition in PaginationState where the TotalItemCountChanged event was not raised when filtered data caused a page index reset. The bug occurred when users were on a high page number (e.g., page 10) and a filter reduced the total item count, requiring a page reset. The recursive call to SetTotalItemCountAsync would see the count was already set and exit early without raising the event, leaving the UI displaying a phantom page state.

Key Changes:

• Added a _hasPendingTotalItemCountChangedEvent flag to track when a page reset triggers a recursive call
• Extracted event raising logic into a new RaiseTotalItemCountChangedAsync helper method
• Added comprehensive test suite to verify the fix and related pagination behaviors

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/Components/QuickGrid/Microsoft.AspNetCore.Components.QuickGrid/src/Pagination/PaginationState.cs	Implements the fix using a flag-based approach to ensure TotalItemCountChanged fires even when recursive calls have the same total count; refactors event raising into a helper method
src/Components/QuickGrid/Microsoft.AspNetCore.Components.QuickGrid/test/PaginationStateTest.cs	Adds new test suite with regression test for the bug and additional tests for page reset behavior, LastPageIndex calculation, event firing, and page preservation scenarios

[javiercn]

Thanks for submitting a PR.

For us to be able to consider taking this change, it should include E2E test coverage in addition to unit tests.

[dariatiurina]

@Pinten84 Thanks a lot for the time and effort you put into this PR. We’ve checked your scenario, and the issue appears to be resolved by the alternative fix in #64661.