Fix TabControl dark mode rendering for Left/Right alignment with vertical text rotation, dark tab strip, and dark border

[SimonZhao888]

Fixes #14109

Proposed changes

• Automatically enable owner-draw mode (TCS_OWNERDRAWFIXED) for vertical tabs (Left/Right alignment) when dark mode is enabled
• Implement DrawDarkModeTab() method with proper vertical text rotation using Graphics.DrawString (GDI+) and very dark/black backgrounds
• Modified OnDrawItem() to automatically render vertical tabs in dark mode when user hasn't set custom draw handler
• Implement 90-degree text rotation for vertical tabs (Left alignment: -90°, Right alignment: +90°)
• Add WM_ERASEBKGND message handling to paint tab strip background dark for vertical tabs
• Add WM_PAINT message handling to fill thick dark border areas around TabPage content for vertical tabs
• Horizontal tabs (Top/Bottom alignment) continue using DarkMode::FileExplorerBannerContainer theme
• Only applies when DrawMode is not explicitly set to OwnerDrawFixed and no DrawItem event handler is attached

Root Cause: Windows standard themes (DarkMode::FileExplorerBannerContainer and DarkMode_Explorer) do not support dark mode rendering for tab controls with the TCS_VERTICAL style (Left/Right alignment). After testing multiple theme-based approaches, an owner-draw solution was implemented.

Technical Implementation: Uses Graphics.DrawString instead of TextRenderer.DrawText because TextRenderer is GDI-based and doesn't respect Graphics rotation transforms, while Graphics.DrawString uses GDI+ and properly honors rotation. Custom WM_PAINT handling fills 3-pixel thick border areas (top, bottom, left, right) around TabPage content area to completely cover the native Windows light border with dark color matching the tab strip background.

Customer Impact

• TabControl tabs with Left/Right alignment now render completely in dark mode with very dark/black backgrounds, properly rotated vertical text, dark tab strip area, and dark border around TabPage content instead of remaining white with horizontal text and light borders
• Affects applications using Application.SetColorMode(SystemColorMode.Dark) with vertically-aligned tab controls
• Backward Compatible: User's explicit DrawMode.OwnerDrawFixed settings and custom DrawItem event handlers are preserved
• Automatic dark mode rendering only activates when neither explicit draw mode nor custom handlers are set
• TabPage content area maintains normal background color (not affected by tab button drawing, tab strip background, or border drawing)

Regression?

• No

Risk

• Low: Automatic owner-draw only activates under specific conditions (dark mode enabled, vertical alignment, no user customization)
• Horizontal tab behavior unchanged - continues using standard theme approach
• Preserves all existing user customization options
• WM_ERASEBKGND and WM_PAINT handling includes safety checks (null validation, try-catch) to prevent crashes

Screenshots

Before

Left and Right alignments show white tabs in dark mode (highlighted in red boxes):

After

Manual testing on Windows required - Vertical tabs (Left/Right alignment) should now render with:

• Very dark/black tab backgrounds: Selected tab #252526, Normal tab #1C1C1C, Border #333333
• Dark tab strip background: #2D2D30 (the area where tabs are placed)
• Dark border around TabPage content area: 3-pixel thick filled areas in #2D2D30 (matching tab strip color)
• Light gray text: #F1F1F1
• Text rotated vertically: -90° for Left alignment (bottom-to-top), +90° for Right alignment (top-to-bottom)
• TabPage content area maintains normal background color

Test methodology

• Manually

Accessibility testing

Test environment(s)

• 11.0.0-alpha.1.25619.109

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

This PR updates TabControl dark-mode rendering for Left/Right (vertical) tab alignment by enabling automatic owner-draw and adding custom background/border painting paths, addressing a gap in Windows theming support for TCS_VERTICAL in dark mode (Fixes #14109).

Changes:

• Automatically sets TCS_OWNERDRAWFIXED for vertical tabs when dark mode is enabled (unless explicitly owner-drawn by the user).
• Adds default dark-mode vertical tab rendering with rotated text via Graphics.DrawString.
• Adds WM_ERASEBKGND / WM_PAINT handling to darken the tab strip background and cover native light borders for vertical tabs.

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