Fix ThreadContext CurrentForm to reflect ApplicationContext MainForm changes

[abineshPalanisamy]

Fixes #3260

Proposed changes

• Update Application.ThreadContext.CurrentForm to resolve from ApplicationContext.MainForm when no temporary form override is active.
• Add a temporary current-form override for non-main message loops.
• Restore the previous current-form override after nested or modal message loops complete.
• Ensure CurrentForm reflects changes to ApplicationContext.MainForm without requiring explicit synchronization from ApplicationContext.
• Preserve modal dialog behavior by allowing modal loops to temporarily override CurrentForm.
• Add regression test coverage for CurrentForm updates when ApplicationContext.MainForm changes.

Root cause

• ApplicationContext.MainForm is allowed to change at runtime.
• Application.ThreadContext maintained a cached CurrentForm reference.
• CurrentForm could become stale after ApplicationContext.MainForm was reassigned.
• Because of this, internal message-loop logic could continue using the old form.

Customer Impact

• Ensures ThreadContext.CurrentForm reflects the latest ApplicationContext.MainForm.
• Prevents stale form references after MainForm is changed.
• Improves correctness for applications that dynamically replace their main form.
• Avoids incorrect behavior when the old main form is closed after switching to a new main form.
• Preserves modal dialog behavior by keeping temporary modal form overrides intact.

Regression?

• No

Risk

• Low.
• The change is internal to Application.ThreadContext.
• No public API behavior is changed.
• ApplicationContext.MainForm continues to behave as documented.
• Modal-loop behavior is preserved through temporary current-form overrides.

Screenshots

• Not applicable. No UI rendering changes.

Test methodology

• Reproduced the issue by changing ApplicationContext.MainForm after the message loop was initialized.
• Verified that before the fix:
  • ApplicationContext.MainForm changed to the new form,
  • ThreadContext.CurrentForm continued to reference the old form.
• Verified that after the fix:
  • ThreadContext.CurrentForm reflects the updated ApplicationContext.MainForm.
  • Setting ApplicationContext.MainForm to null is reflected by CurrentForm.
  • Multiple MainForm changes are reflected correctly.
  • Modal form override behavior is preserved.
• Added unit test coverage for:
  • ThreadContext_CurrentForm_ReflectsChangedMainForm
  • ThreadContext_CurrentForm_ReflectsNullMainForm
  • ThreadContext_CurrentForm_ReflectsMultipleMainFormChanges
  • ThreadContext_CurrentForm_OverrideTakesPriorityOverMainForm

Accessibility testing

Test environment(s)

• 11.0.100-preview.3.26170.106

Microsoft Reviewers: Open in CodeFlow