Fix Issue - [Dark Mode] The BackColor of the ProgressBar control should not be changed - #15244

Fixes #11938

Proposed changes

• Use SystemColors.ControlDarkDark as the native ProgressBar background in Dark Mode, regardless of a custom BackColor value.
• Apply the fixed background during control creation, handle creation, and system-color updates; prevent subsequent BackColor changes from updating the native background in Dark Mode.
• Add ProgressBar_DarkMode_CustomBackColor_DoesNotLeakThroughVisually to verify that the managed property retains the assigned color while the native background uses the dark-mode color.

Customer Impact

• Setting ProgressBar.BackColor no longer produces an unexpected custom-colored background in Dark Mode, matching the behavior reported for Classic Mode with visual styles enabled.
• The assigned BackColor property value and change notifications remain available. Non-dark-mode background handling is unchanged.

Regression?

• No. The issue was reported against the newly introduced Dark Mode feature, rather than previously working behavior in an earlier release.

Risk

• Low, scoped to ProgressBar background rendering in Dark Mode. Applications relying on custom backgrounds in Dark Mode will intentionally render differently. High Contrast and visual behavior still require manual verification; the new test bypasses its assertions in High Contrast.

Screenshots

Before

After

Test methodology

• Added a test that enables Dark Mode, assigns Color.Red , creates the control handle, and checks that BackColor remains red while the native background equals SystemColors.ControlDarkDark . The test restores Classic Mode afterward.
• Manual reproduction, post-handle property changes, handle recreation, system-color changes, and Classic Mode/High Contrast comparisons have not been verified in this session.

Accessibility testing

NA

Test environment(s)

• Attempted validation on Windows, OS version 10.0.26200, win-x64, using the repository-local .NET SDK 11.0.100-preview.6.26359.118.
• Original issue environment: .NET SDK 9.0.100-rc.1.24418.2 with privately built WinForms assemblies from the main branch.

Microsoft Reviewers: Open in CodeFlow