Group List not shown when multiple items are selected and editing ‘Group’ property in ListViewItem Collection Editor

[Lydia-Shi]

• .NET Core Version: .Net 6.0.100-alpha.1.20506.3

• Have you experienced this same bug with .NET Framework?: Yes

• More info: It's not a regression issue, it also occurs in .Net core 3.1.

Problem description: Group List not shown when multiple items are selected and editing ‘Group’ property in ListViewItem Collection Editor

Expected behavior: Group list should be displayed.

Minimal repro:

1. Create a Winforms core project
2. Drag and drop a ListView control to Form designer, open its smart tag and click "Edit Groups"
3. In ListViewGroup Collection Editor, click "Add" button to add two groups, then click Ok button
4. Focus on ListView control again, open its smart tag and click "Edit items"
5. In ListViewItem Collection Editor, click "Add" button to add two items
6. Select these two items, click the drop-down button of Group property in the right table, observe the drop-down box

[RussKie]

The fix would likely involve changes to ListViewGroupConverter to handle the scenario where multiple ListViewItems are selected.