Fix ComboBox drop-down button incorrectly uses BackColor

[LeafShi1]

Fixes #14849

Root Cause

The NET11 ModernComboAdapter treated ComboBox.BackColor as the base color for the drop-down button. As a result, assigning a custom field background also tinted the button, unlike Classic mode where the button remains system-rendered.

Proposed changes

• Update the NET11 ComboBox renderer so the drop-down button no longer derives its surface color from ComboBox.BackColor.

Customer Impact

• Setting BackColor customizes only the editable field, the drop-down button retains a neutral system appearance.

Regression?

• No

Risk

• Minimal

Screenshots

Before

Editable field and drop-down button both use/tint from BackColor.

After

Only the editable field uses BackColor, the drop-down button retains neutral system chrome.

DarkMode:

Light mode:

Test methodology

• Manually

Test environment(s)

• .net 11.0.0-rc.1.26405.103

Microsoft Reviewers: Open in CodeFlow

[Copilot]

Pull request overview

Adjusts the NET11 modern ComboBox renderer so the drop-down button no longer picks up the control’s BackColor, aligning the visual behavior more closely with Classic mode expectations (custom field background without tinting the button chrome).

Changes:

• Switch drop-down button rendering to use a fixed “base” surface color instead of GetEffectiveBackColor(comboBox).
• Add a helper (GetDropDownButtonBaseColor) that selects a system-derived base color (with a dark-mode variant).

Suppressed comments (1)

src/System.Windows.Forms/System/Windows/Forms/Controls/ComboBox/ComboBox.ModernComboAdapter.cs:208

• This change alters rendering behavior (drop-down button background no longer derives from ComboBox.BackColor), but there doesn’t appear to be a regression test asserting the button region ignores a custom BackColor under VisualStylesMode.Net11. There are already bitmap-based tests in ComboBoxTests.cs for modern adapter rendering, so adding a similar test (sampling pixels within _buttonBounds) would help prevent regressions of #14849.

            Color background = GetDropDownButtonBaseColor(comboBox);
            Color buttonColor = comboBox._mousePressed
                ? PopupButtonColorMath.Blend(
                    background,
                    Application.SystemVisualSettings.AccentColor,

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[SimonZhao888]

Looks good to me!