Fix ToolStripDropDownMenu scroll boundary handling to prevent out-of-range and unintended dropdown dismissal

[LeafShi1]

Fixes #6650

Proposed changes

• Use explicit visibility checks (fully visible / intersects viewport) instead of edge-based Rectangle.Contains(...).
• Update scroll button state using item index positions (first/last available item), not pixel min/max.
• Add an early return in ScrollInternal(bool up) when the target scroll button is already disabled.
• Treat ToolStripScrollButton as navigation-only (SupportsItemClick = false) so scrolling does not trigger dropdown auto-close behavior.
• Add regression tests for:
  • down button disabled at end of list,
  • no extra scroll when item is already at boundary,
  • scroll buttons not participating in item-click dismissal.

Customer Impact

• Fixes scenario where scrolling to the last dropdown item could still allow another down action and hit debug-time failures.
• Fixes incorrect down-arrow enabled state at bottom boundary.
• Reduces risk of dropdown collapsing during boundary scroll interaction.
• Improves reliability and maintainability of scroll behavior for menus with variable item visibility.

Regression?

• No

Risk

• Minimal

Screenshots

Before

The ToolStripScrollDownButton is still enabled when displaying the last dropdown item, and in this case, an exception pops up when clicking that ToolStripScrollDownButton again

After

When go through the last dropdown item, the ToolStripScrollDownButton is disabled

AfterChanges.mp4

Test methodology

• Manually and unit test

Test environment(s)

• .net 11.0.0-preview.4.26215.121

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

Fixes ToolStripDropDownMenu scrolling boundary behavior so scroll buttons reflect actual scrollability, prevent out-of-range scrolling at boundaries, and avoid unintended dropdown dismissal when interacting with scroll buttons.

Changes:

• Replaces edge-based Rectangle.Contains(...) checks with explicit “fully visible” / “intersects viewport” checks.
• Updates scroll button enable/disable logic and adds early-return guards to avoid invalid scrolling paths.
• Marks ToolStripScrollButton as navigation-only (SupportsItemClick = false) and adds regression unit tests.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/System.Windows.Forms/System/Windows/Forms/Controls/ToolStrips/ToolStripDropDownMenu.cs	Adjusts visibility math, scroll boundary logic, and scroll-button status calculation.
src/System.Windows.Forms/System/Windows/Forms/Controls/ToolStrips/ToolStripScrollButton.cs	Prevents scroll buttons from participating in item-click routing to avoid AutoClose dismissal.
src/test/unit/System.Windows.Forms/ToolStripDropDownMenuTests.cs	Adds/updates unit tests covering scroll boundary and scroll-button click behavior.

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[SimonZhao888]

Please address the Copilot suggestions.