Fix issue 14795: ComboBox with DropDownStyle.Simple renders incorrectly and displays an unexpected vertical scrollbar

[SimonZhao888]

Fixes #14795

Root Cause

When VisualStylesMode is set to Net11, ComboBoxStyle.Simple retains certain "classic/native" geometric and non-client area behaviors, resulting in inconsistencies when overlaid with modern rendering:

• An extra visual "container layer" (including the layer containing the scrollbar) overlaps with the internal list area.
• The vertical allocation between the edit area and the list area is unstable, causing the displayed text to be compressed or clipped.
• The border of the list sub-window conflicts with the modern rounded outer frame, leading to noticeable issues with whitespace at the bottom or overlapping lines.

Proposed changes

• Unified the modern geometric paths for Net11 and Simple modes (including client-side extensions and consistent hit-testing) to eliminate visual interference from outer hierarchy layers.
• Recalculated the layout for the Simple mode's editing and list areas:
  • Increased the editing area height (including an extra 1px) to ensure full font display;
  • Positioned the list area immediately below the editing area, extending it to the bottom of the available space within the outer frame.
• Added a separator line between the editing and list areas (using AccentColor) to improve the visibility of the boundary.
• Removed borders from the list sub-window (removing WS_BORDER / WS_EX_CLIENTEDGE) to maintain a modern aesthetic consistent with the outer rounded-corner frame.
• Applied slight bottom cropping to the visible list area (a visual reduction of approximately 2px) to prevent obscuring the bottom rounded corner.
• Added regression test coverage for: editing area readability, separator line rendering, list border removal, list fill area, and style flag behavior.

Customer Impact

• Significant improvement in visual consistency for Net11 + Simple ComboBox: retains the modern rounded-corner frame while removing unnecessary layering effects.
• Text in the editing area is fully visible, reducing issues where the top or bottom edges of characters were previously clipped.

Regression?

• No

Risk

• Mini

Screenshots

Before

After

Normal

DarkMode

Test methodology

• Manually
• Automated test cases

Test environment(s)

• 11.0.100-preview.5.26302.115

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

This PR addresses WinForms issue #14795 by correcting the Net11 modern-visual-styles rendering/layout of ComboBoxStyle.Simple, eliminating the visual interference that produced an unexpected vertical scrollbar strip and improving edit/list geometry consistency.

Changes:

• Adjusts modern Simple-mode layout to stabilize edit/list sizing, remove list borders, and clip the list bottom to preserve the rounded outer frame.
• Adds an accent-colored divider between the edit area and the list area during modern painting.
• Adds regression tests covering Simple-mode edit/list bounds, divider rendering, list border removal, and CreateParams flags.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
src/test/unit/System.Windows.Forms/System/Windows/Forms/ComboBoxTests.cs	Adds regression tests for modern Net11 + Simple layout, divider rendering, and style flags.
src/System.Windows.Forms/System/Windows/Forms/Controls/ComboBox/ComboBox.ModernComboAdapter.cs	Draws an accent divider between the Simple edit and list areas during modern rendering.
src/System.Windows.Forms/System/Windows/Forms/Controls/ComboBox/ComboBox.Modern.cs	Reworks modern Simple-mode geometry; removes list borders; applies a clip region to avoid rounding artifacts.
src/System.Windows.Forms/System/Windows/Forms/Controls/ComboBox/ComboBox.cs	Tweaks CreateParams (integral height for modern Simple) and applies list border adjustments on handle creation; modernizes NCCALCSIZE/NCHITTEST behavior for Simple.

Comments suppressed due to low confidence (2)

src/test/unit/System.Windows.Forms/System/Windows/Forms/ComboBoxTests.cs:1315

• These assertions only check one-sided bounds (<= / >=), which can let significant misalignment slip through. Using InRange with a small tolerance makes the test better at catching regressions while still allowing minor off-by-one differences.

        Assert.True(listBounds.Left <= control.ModernChromeInsets.Left);
        Assert.True(listBounds.Right >= control.ClientRectangle.Right - control.ModernChromeInsets.Right - 1);

src/System.Windows.Forms/System/Windows/Forms/Controls/ComboBox/ComboBox.Modern.cs:610

• The value "2" is a magic number used to crop the visible list area. Giving it a named constant (even a local const) makes it clearer what the crop represents and helps keep related calculations consistent if it needs adjustment later.

        int bottomShrink = ScaleHelper.ScaleToDpi(2, DeviceDpiInternal);

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