Fix CheckedListBox selection highlight jumping when pressing Space with consecutive empty items

[KSaiSivani]

Fixes #5526

Proposed changes

• Added e.Handled = true; in OnKeyPress override for Space key.
• Prevents the selected index from advancing incorrectly on consecutive empty items.
• Ensures only the selected item toggles its check state.

Customer Impact

• Users pressing Space on consecutive empty items will no longer see the selection highlight jump forward.
• Provides consistent and expected keyboard behavior in CheckedListBox.

Regression?

• No

Risk

• Low: change is limited to Space key handling in CheckedListBox.

Screenshots

Before

Selection jumps forward when pressing Space in a CheckedListBox that contains consecutive empty items.

CheckedListBox_BeforeFix.mp4

After

Selection remains stable, only toggles checked state of the selected item.

CheckedListBox_AfterFix.mp4

Test methodology

• Manual validation with and without fix:
  • Before fix → pressing Space caused the selected index to change incorrectly.
  • After fix → pressing Space no longer changes the selected index.

Test environment(s)

OS: Windows
SDK: 11.0.100-preview.3.26170.106

Microsoft Reviewers: Open in CodeFlow

[SimonZhao888]

@KSaiSivani, could you add an automated regression test for this scenario (consecutive empty/null-display items + Space key) to lock in the behavior? Since this bug has existed across multiple versions, a focused test would reduce the chance of future regressions.

[Copilot]

Pull request overview

Fixes a CheckedListBox keyboard interaction bug where pressing Space on consecutive empty/null items could cause the selection highlight to advance unexpectedly (#5526). The change ensures the Space key is treated as handled after toggling the selected item’s check state, preventing additional default processing that can move selection.

Changes:

• Mark Space keypress as handled in CheckedListBox.OnKeyPress after invoking LbnSelChange().
• Prevent unintended selection index changes when Space is pressed on consecutive empty/null items.
• Preserve expected behavior: only the currently selected item toggles its check state.

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[KSaiSivani]

@KSaiSivani, could you add an automated regression test for this scenario (consecutive empty/null-display items + Space key) to lock in the behavior? Since this bug has existed across multiple versions, a focused test would reduce the chance of future regressions.

@SimonZhao888, The automated regression test cases for the consecutive empty/null item + Space key scenario have been committed. Verified that the tests pass with the fix and fail without the fix, ensuring regression coverage and reducing the chance of future regressions.

[SimonZhao888]

@SimonZhao888, The automated regression test cases for the consecutive empty/null item + Space key scenario have been committed. Verified that the tests pass with the fix and fail without the fix, ensuring regression coverage and reducing the chance of future regressions.

There are some build errors; please fix them first.

D:\a\_work\1\s\src\test\unit\System.Windows.Forms\CheckedListBoxTests.cs(867,17): error CS0122: 'CheckedListBox.OnKeyPress(KeyPressEventArgs)' is inaccessible due to its protection level [D:\a\_work\1\s\src\test\unit\System.Windows.Forms\System.Windows.Forms.Tests.csproj]
D:\a\_work\1\s\src\test\unit\System.Windows.Forms\CheckedListBoxTests.cs(883,17): error CS0122: 'CheckedListBox.OnKeyPress(KeyPressEventArgs)' is inaccessible due to its protection level [D:\a\_work\1\s\src\test\unit\System.Windows.Forms\System.Windows.Forms.Tests.csproj]