Fix ContextMenuStrip submenu placement across monitors

[sanskarIN]

Summary

Fixes #11289.

ContextMenuStrip submenus can open on the adjacent monitor when the parent menu is close to a monitor boundary, particularly in multi-monitor configurations with different DPI settings.

This change updates ToolStripDropDownItem.DropDownDirection so horizontal submenus compare their available space on the monitor containing the parent item and choose the direction that provides more visible area on that monitor.

Changes

• Preserve the existing parent-item collision handling.
• Determine the working area of the monitor containing the parent menu item.
• Compare the visible area available for the current horizontal dropdown direction with the opposite direction.
• Select the direction that keeps more of the submenu on the current monitor.
• Leave vertical dropdown-direction behavior unchanged.

Validation

Tested the affected behavior with a multi-monitor Windows configuration matching the conditions reported in #11289.

Verified:

• A submenu near the right edge remains on the current monitor when opening to the right would cross the monitor boundary.
• A submenu near the left edge can switch to the right when that provides more space.
• The menu continues to work normally on a single-monitor configuration.
• Existing dropdown behavior away from monitor boundaries is unchanged.

Related issue

Fixes #11289.

Microsoft Reviewers: Open in CodeFlow

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 37.24166%. Comparing base (75072a6) to head (150f540).

Additional details and impacted files

@@              Coverage Diff              @@
##                main      #15157   +/-   ##
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  Coverage   37.24166%   37.24166%           
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  Files            246         246           
  Lines           9774        9774           
  Branches        1029        1029           
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  Hits            3640        3640           
  Misses          5970        5970           
  Partials         164         164           

Flag	Coverage Δ
Debug	37.24166% <ø> (ø)
production	39.36526% <ø> (ø)
test	20.64923% <ø> (ø)
unit	39.36526% <ø> (ø)

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[Copilot]

Copilot review overview

🟡 Changes recommended

Placement bounds are inconsistent, root menu placement is not addressed, and regression coverage is missing.

Get a fresh assessment by requesting another Copilot review.

Review effort: Lite
Findings: 1 · 1

Open (2)

• Use owner-aware bounds for submenu direction comparison · New
• Add edge-placement regression tests for both directions · New

What changed in this PR

This pull request improves horizontal ContextMenuStrip submenu placement near monitor boundaries.

Changes:

• Preserves parent-item collision handling.
• Selects the horizontal direction with more visible space.
• Leaves vertical dropdown behavior unchanged.

File	Summary
src/​System.Windows.Forms/​System/​Windows/​Forms/​Controls/​ToolStrips/​ToolStripDropDownItem.cs	Adds monitor-aware horizontal submenu direction selection.

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