TextBox and MaskedTextBox.GetPositionFromCharIndex() and MaskedTextBox.GetCharIndexFromPosition() methods return the incorrect values

[John-Qiao]

• Have you experienced this same bug with .NET Framework?: No

OS: RS4 X64
VS: Dev16 16.2.0 Preview 1.0 [28913.52.d16.2]
.NET Core: 3.0.100-preview6-011867 from Master branch

More Info:
This issue can repro in TextBox control too, but RichTextBox control works well.

Steps to reproduce:

1. Install the .NET Core 3.0 SDK from https://github.com/dotnet/core-sdk for Master branch.
2. Open the attached ‘.NETCoreTestcase’ .NET Core app in VS.
3. Start it and observe the returned values from GetPositionFromCharIndex() and GetCharIndexFromPosition() methods in debugging result.
   NETCoreTestcase.zip

Actual:
Both of the returned values are 0.

Expected:
These two methods should return the correct values, like it works in .NET Framework.

[merriemcgaw]

@John-Qiao Can you see if this still repros and tell us if this is a regression from an earlier .NET Core Build?

[RussKie]

I think this is caused by a regression introduced in #791 c1a3b67.
More info: c1a3b67#r34050026

I suspect we need to perform a thorough review of that change, or revert it back.

[John-Qiao]

@merriemcgaw @RussKie This issue still repros in the latest .NET Core 3.0.100-preview7-012578 version from Master branch. And in my test result, it's a regressions issue from .NET Core 3.0.100-preview5-011534 version.

[hughbe]

Yes. It looks like the EM_* messages are incorrect. Bizarely they were incorrect before my change (when defined in RichTextBoxConstants.cs) but it was being used in different places.

https://github.com/dotnet/winforms/pull/791/files#diff-aafd5fb74483abd9f7febf2c487e1385L60