Fix issue 14489: ToolStrip (TabStop=true) intercepts arrow keys after tab navigation, preventing TreeView from handling them

[SimonZhao888]

Fixes #14489

Proposed changes

• Restrict ModalMenuFilter.SetActiveToolStrip(this) in ToolStrip.WndProc(WM_SETFOCUS) to focus transitions originating from ToolStrip context only.
• Preserve ToolStrip-to-ToolStrip activation behavior (Cannot use down to expand ToolStripDropDownButton in toolStrip2 after using right/Left to collapse ToolStripDropDownButton in toolStrip1 #12916), while preventing accidental menu activation on generic Tab focus ([Accessibility] ToolStrip (TabStop=true) intercepts arrow keys after tab navigation, preventing TreeView from handling them (.NET 10 regression) #14489).
• Scope is focused to menu-activation gating logic; no layout/rendering/overflow behavior changes intended.

Customer Impact

• ToolStrip (TabStop=true) no longer intercepts arrow keys after tab navigation, and not preventing TreeView from handling them.

Regression?

• Yes

Risk

• minimal

Screenshots

Before

ToolStripIssue.mp4

After

2026-04-24.141412.mp4

Test methodology

• Manually

Test environment(s)

• .net 11.0.0-preview.2.final

Microsoft Reviewers: Open in CodeFlow

[SimonZhao888]

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Pull request overview

This PR addresses a .NET 10/11 regression where a ToolStrip with TabStop=true can remain “active” in ModalMenuFilter after tab-navigation, causing arrow keys to be intercepted (e.g., preventing TreeView from handling them). The change narrows when a ToolStrip becomes active in menu mode and adds logic to clear the active ToolStrip when focus leaves ToolStrip context.

Changes:

• Gate ModalMenuFilter.SetActiveToolStrip(this) in ToolStrip.WndProc(WM_SETFOCUS) so it only runs for focus transitions originating from ToolStrip context (preserving #12916 behavior while avoiding #14489).
• On ToolStrip.OnLostFocus, remove the ToolStrip from the modal menu filter and potentially exit menu mode when focus moves outside ToolStrip context.

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[LeafShi1]

Thanks for the fix, @SimonZhao888! The approach of gating SetActiveToolStrip on ToolStrip-context focus transitions and clearing on WM_KILLFOCUS makes sense. I have a few observations below.