DataGrid with ComboBoxColumn Selection Issue

[joru1407]

Describe the bug

DataGridComboBoxColumn's behave unexpected when using them in a DataGrid the MVVM way and scrolling around trough the rows of the DataGrid after MahApps Version 2.0.1. In the first X rows the first List Entry of the ComboBoxes get's randomly selected. This change also affects the MVVM Objects behind the DataGrid. If you scroll down and up a little bit, this wrong selection is appearing and diasappearing. WIth Version 2.0.1 everything ist working as expected.

Steps to reproduce

1. .NET Core 3.1 Project with MahApps 2.0.1 and MahApss.Window
2. Create a Observable Collection containing Objects and bind it to a DataGrid
3. DataGrid shoud contain a bound TextBoxColumn and a ComboBoxColumn
4. ItemSource of the COmboBoxColumn is a StaticResource, SelectedValuePath and SelectedValueBinding are set
5. Fill the DataGrid and the ComboBox List with Objects during runtime (300 f.e.)
6. Everything works as expected (in every Row theres's nothing selected in the ComboBox, and you can select it)
7. Upgrade MahApps to a newer Version trough NuGet
8. Now there is randomly the first Value ov the ComboBox Selected in the upper 10 or 12 rows f.e.
9. If you scroll up- or down the DataGrid this wrong selection is appearing and disappearing

This Problem seems not to be related with any other code, Events or the MVVM Models itself. JUst replacing the MahApps Version with an older one solves the problem.

MahApps.Metro version: v2.1.0 or newer
Windows build number: Win10 2004
Visual Studio: 2019 16.8.2
Target Framework: .Net Core 3.1

## Screenshots

![](https://user-images.githubusercontent.com/65429097/100463961-fd88e180-30cc-11eb-826f-10ae77db6d4b.gif)

[timunie]

@joru1407 may I ask you to provide a sample App via github Repro showing your issue? I don't see the issue on my side right now.

Thanks and happy coding
Tim

[joru1407]

@timunie Attached you can find a simple App showing the Problem.
Just select a value in the first row's ComboBox and scroll up and down a little bit ;-)

If you just downgrade the NuGet Package of MahApps to 2.0.0 everything work's fine.

Greetings,
Joshua

DataGridIssue.zip

[punker76]

@joru1407 I got it, it's a bug in our default ComboBox style for the DataGrid column.

[joru1407]

@punker76 Wow, that was quick! Thank you very much, Joshua