GetCharacterIndexFromPoint does not work as intended if HorizontalContentAlignment is set to Center

[rampaa]

Description

If the HorizontalContentAlignment of a TextBox is set to "Center" (or to "Right"), GetCharacterIndexFromPoint does not return the correct character index corresponding to a given point and instead returns -1.

Reproduction Steps

See the following demo:
GetCharacterIndexFromPointBugDemo.zip

Expected behavior

GetCharacterIndexFromPoint should return the correct character index from a given point even if the HorizontalContentAlignment is set to Center.

Actual behavior

GetCharacterIndexFromPoint returns -1 if HorizontalContentAlignment is set to Center/Right

Regression?

No response

Known Workarounds

No response

Impact

No response

Configuration

.NET 6/7
Windows 10 x64

Other information

No response

[huiliu-code]

Hi,@rampaa. I encountered errors when unzipping your zip file. Could you provide a new zip file that can reproduce the problem?

[rampaa]

https://github.com/rampaa/GetCharacterIndexFromPointBugDemo

[miloush]

OK found it, the important part is that GetCharacterIndexFromPoint is called with snapToText: false.

In that case TextBoxView.GetLineIndexFromPoint cannot find the line:

wpf/src/Microsoft.DotNet.Wpf/src/PresentationFramework/MS/Internal/documents/TextBoxView.cs

Lines 1642 to 1646 in f5be928

	// Returns the index of a line containing point, or -1 if no such
	// line exists. If snapToText is true, the closest match is returned.
	//
	// Point must be in document space.
	private int GetLineIndexFromPoint(Point point, bool snapToText)

and the whole character finding is cancelled.

wpf/src/Microsoft.DotNet.Wpf/src/PresentationFramework/MS/Internal/documents/TextBoxView.cs

Lines 612 to 614 in f5be928

	if (lineIndex == -1)
	{
	position = null;

So a workaround is to use snapToText: true.

I think the fix should be in GetLineIndexFromPoint to take into account line alignment.

[dipeshmsft]

@rampaa @miloush , closing this issue as the fix has been merged in the main branch

[rampaa]

@dipeshmsft will the fix be backported to .NET 6?

[pchaurasia14]

@rampaa - We'll need to discuss with shiproom if this can be backported to .NET 6/7.

[rampaa]

@pchaurasia14 thanks for your reply! I've developed an application that makes extensive use of GetCharacterIndexFromPoint so this bug is a real headache for me and for my user base. Would really appreciate a .NET 6 backport.