Fix RichTextBox border artifacts when BorderStyle is None under VisualStylesMode.Net11

[abineshPalanisamy]

Fixes #15055

Proposed changes

• Fix borderless RichTextBox rendering under VisualStylesMode.Net11.
• Prevent unwanted thin border artifacts from appearing on the top and left edges when BorderStyle = None.
• Adjust the NET11 rendering path to preserve the expected borderless appearance.
• Add regression test coverage for borderless rendering scenarios.

Root cause

• The issue was reproducible only when VisualStylesMode = Net11.
• The issue was not reproducible in Classic mode.
• The NET11 TextBoxBase.OnNcPaint rendering path introduced unintended border artifacts when BorderStyle = None.
• As a result, thin border lines appeared on the top and left edges of a borderless RichTextBox.
• Borderless controls were not rendered consistently with their expected appearance.

Customer Impact

• RichTextBox controls configured with BorderStyle.None now render correctly under VisualStylesMode.Net11.
• Removes unwanted top and left border artifacts from borderless controls.
• Provides a rendering experience consistent with the expected borderless appearance.

Regression?

• No

Risk

• Low.
• The change is limited to the NET11 rendering path in TextBoxBase.OnNcPaint.
• Existing behavior for FixedSingle and Fixed3D remains unchanged.
• Classic rendering behavior is unchanged.
• Added regression test coverage for borderless rendering scenarios.

Screenshots

Before

After

Test methodology

• Reproduced the issue using a RichTextBox with:
  • BorderStyle = None
  • VisualStylesMode = Net11
• Verified that thin border lines were visible on the top and left edges before the fix.
• Verified that the border artifacts are no longer visible after the fix.
• Verified rendering behavior in both focused and unfocused states.
• Verified runtime border-style transition scenarios.
• Verified that FixedSingle rendering remains unchanged.
• Verified that Fixed3D rendering remains unchanged.
• Added regression test coverage for borderless rendering scenarios.

Accessibility testing

Test environment(s)

• 11.0.100-rc.1.26420.103
• VisualStylesMode.Net11

Microsoft Reviewers: Open in CodeFlow

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 37.24166%. Comparing base (1ab988e) to head (c4c77a2).

Additional details and impacted files

@@              Coverage Diff              @@
##                main      #15141   +/-   ##
=============================================
  Coverage   37.24166%   37.24166%           
=============================================
  Files            246         246           
  Lines           9774        9774           
  Branches        1029        1029           
=============================================
  Hits            3640        3640           
  Misses          5970        5970           
  Partials         164         164           

Flag	Coverage Δ
Debug	37.24166% <ø> (ø)
production	39.36526% <ø> (ø)
test	20.64923% <ø> (ø)
unit	39.36526% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.