Fix editable ComboBox Shift+Tab keyboard trap in WPF Gallery (ADO 3019001)

[wnvko-msft]

Summary

Fixes a keyboard trap on the Basic Input -> ComboBox page of WPF Gallery. When focus is on the editable ComboBox's edit field, pressing Shift+Tab does not move focus to the previous control, trapping keyboard users.

Fixes ADO 3019001.

Root cause

The .NET Fluent theme's DefaultComboBoxStyle IsEditable=True trigger only swaps the control template - unlike the classic/Aero theme, it never sets IsTabStop="False" on the ComboBox. As a result, an editable ComboBox exposes two tab stops: the ComboBox itself and its inner PART_EditableTextBox. Shift+Tab from the edit field lands on the ComboBox container, which redirects focus back into the edit field, so focus never leaves the control. (Forward Tab happens to escape; Shift+Tab does not.)

Fix

Set IsTabStop="False" on the editable ComboBox in Views/BasicInput/ComboBoxPage.xaml, leaving only the inner edit field as a tab stop (mirroring the classic-theme behavior). Shift+Tab now moves focus to the previous control.

Scope / notes

• Sample-only change; one XAML file touched.
• The underlying Fluent-theme defect still affects other editable ComboBoxes and may warrant a separate upstream fix in dotnet/wpf.

Testing

• Verified Shift+Tab from the editable ComboBox now moves focus to the previous control.
• dotnet build succeeds (0 errors).

Accessibility

• MAS 2.1.1 – Keyboard / WCAG 2.1.1

Microsoft Reviewers: Open in CodeFlow