Text Controls don't lose highlighting when unselected (TextBox / RichEditBox)

[AlexanderBlackman]

Describe the bug

Selecting text highlights the text, but unselecting it still maintains the highlighted background colour. Interestingly foreground colour doesn't persist after unselection. This weird behaviour only occurs after the first visual row, which behaves normally. Only multiline selections that include the first row behave normally. This is a problem in both TextBox and RichEditBox.

All highlighting is lost when the control regains focus. (As expected)

Steps to reproduce the bug

Loaded the normal WinUI 3 app template. Added a RichEditBox and a TextBox.

Highlight some text after the first visual row. (it doesn't matter if done by the keyboard or mouse) Highlighted some other text to clicked elsewhere, the highlighting background colour still persists in the now unselected text.

No custom styling has been applied. Light/Dark theme doesn't change anything. This bug occurs with both the default and newest WASDK

Expected behavior

Selected text should lose its highlighting if unselected. This should happen no matter what line it is.

Screenshots

NuGet package version

WinUI 3 - Windows App SDK 1.6.1: 1.6.240923002

Windows version

Windows 11 (22H2): Build 22621

Additional context

No response

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• TextBox selection is broken when there are multiple lines (#9965), similarity score: 0.82

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[AlexanderBlackman]

Further testing.

Partial success

• If I locally set the text background to another background colour via ITextRange.CharacterFormat.BackgroundColor = (this.Background as SolidColorBrush)?.Color ?? Colors.Transparent; selection highlighting doesn't persist.

Failures

• Calling InvalidateArrange() nor Document.ApplyDisplayUpdates() in a SelectionChanged event have no positive effect.
• Setting the RichEditBox Background property doesn't change the weird behaviour.
• Putting this in my Loaded event doesn't work, var myFormat = Document.GetDefaultCharacterFormat(); myFormat.BackgroundColor = (this.Background as SolidColorBrush)?.Color ?? Colors.Transparent;