Fix issue 14795: ComboBox with DropDownStyle.Simple renders incorrectly and displays an unexpected vertical scrollbar by SimonZhao888 · Pull Request #14827 · dotnet/winforms

SimonZhao888 · 2026-07-30T10:06:45Z

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

…ly and displays an unexpected vertical scrollbar

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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LeafShi1 · 2026-08-11T07:30:22Z

+            if (!_integralHeight
+                || (UsesModernComboAdapter
+                    && DropDownStyle == ComboBoxStyle.Simple
+                    && !_integralHeightExplicitlySet))


This “explicitly set” state is not preserved by WinForms designer serialization. IntegralHeight still has [DefaultValue(true)], so the designer omits an assignment when its value is true; after reopening the form, _integralHeightExplicitlySet is therefore false and CBS_NOINTEGRALHEIGHT is silently enabled. Two controls with the same serialized/public value can behave differently based only on setter history. This needs value-based, serializable semantics (or custom reset/serialization handling), rather than an unpersisted flag.

LeafShi1 · 2026-08-11T07:30:23Z

+        int bottomShrink = ScaleHelper.ScaleToDpi(
+            ModernSimpleBottomCropLogicalPixels,
+            DeviceDpiInternal);
+        int visibleHeight = Math.Max(1, targetBounds.Height - bottomShrink);


The bottom crop is still applied twice. ComputeModernComboTargetState already subtracts ModernSimpleBottomCropLogicalPixels (and the divider thickness) when calculating simpleListBottom, then this region removes the same crop again. At 96 DPI the intended ~2px crop becomes roughly 5px (2 + divider + 2), potentially clipping the last row or the bottom of the scrollbar. Please apply the crop in only one place.

Fix issue 14795: ComboBox with DropDownStyle.Simple renders incorrect…

c4444e3

…ly and displays an unexpected vertical scrollbar

SimonZhao888 requested a review from a team as a code owner July 30, 2026 10:06

SimonZhao888 requested a review from Copilot July 30, 2026 10:06

dotnet-policy-service Bot assigned SimonZhao888 Jul 30, 2026

Copilot started reviewing on behalf of SimonZhao888 July 30, 2026 10:07 View session

Copilot AI reviewed Jul 30, 2026

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Comment thread src/test/unit/System.Windows.Forms/System/Windows/Forms/ComboBoxTests.cs

Comment thread src/test/unit/System.Windows.Forms/System/Windows/Forms/ComboBoxTests.cs

Comment thread src/System.Windows.Forms/System/Windows/Forms/Controls/ComboBox/ComboBox.Modern.cs Outdated

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