Fix disabled RichTextBox multiline text rendering in Dark Mode

[ricardobossan]

Fixes #14203

Root Cause

• When a RichTextBox is disabled in Dark Mode, the custom painting code used ClientRectangle instead of the proper text formatting rectangle, causing text to render incorrectly at the control's edge rather than within the text area bounds.
• The EndEllipsis flag was incompatible with multiline text, causing premature truncation.

Proposed changes

• Use EM_GETRECT to get the proper text formatting rectangle that accounts for borders and padding.
• Replace EndEllipsis with TextBoxControl flag for proper multiline text rendering.

Customer Impact

• Disabled RichTextBox controls now display multiline text correctly in Dark Mode.

Regression?

• No

Risk

• Minimal

Screenshots

Before

After

Test methodology

• Manual

Test environment(s)

• 11.0.100-alpha.1.25618.104

Microsoft Reviewers: Open in CodeFlow

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 77.17093%. Comparing base (ff0d8d0) to head (800a8da).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                 @@
##                main      #14216         +/-   ##
===================================================
+ Coverage   77.15694%   77.17093%   +0.01398%     
===================================================
  Files           3279        3279                 
  Lines         645347      645059        -288     
  Branches       47721       47722          +1     
===================================================
- Hits          497930      497798        -132     
+ Misses        143729      143570        -159     
- Partials        3688        3691          +3     

Flag	Coverage Δ
Debug	77.17093% <0.00000%> (+0.01398%)	⬆️
integration	18.99442% <0.00000%> (-0.00213%)	⬇️
production	52.03286% <0.00000%> (+0.00603%)	⬆️
test	97.40560% <ø> (+0.00001%)	⬆️
unit	49.48101% <0.00000%> (+0.01527%)	⬆️

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[SimonZhao888]

Why don't we use ClientRectangle? I think what really makes a difference is the modification to TextFormatFlags.EndEllipsis, because it indicates that when the text is too long, it will no longer be omitted with "...".

[Copilot]

Pull request overview

This PR fixes a rendering issue with disabled RichTextBox controls in Dark Mode where multiline text was not displayed correctly. The root cause was using ClientRectangle instead of the proper text formatting rectangle, and using incompatible text formatting flags.

Changes:

• Use EM_GETRECT to retrieve the correct text formatting rectangle that accounts for borders and padding
• Replace EndEllipsis flag with TextBoxControl flag for proper multiline text rendering