Description

When using a DataGrid with Extended SelectionMode which has an initial selection, an anchor selection does not work as expected.

Reproduction Steps

Repro project with DataGrid and ListBox
E.g. in the example project.

• Item 1 is initially selected
• Shift-Click on Item 3

Additional Scenario:

• Now Select Item 1 via the ListBox
• Shift Click on Item 5 in the DataGrid

Expected behavior

• Item 1, Item 2 and Item 3 are selected after Shift-Click.

This expected behavior is the behavior that can be observed in the ListBox.
In the DataGrid this behavior occurs only when the initial selection was done by user input.

Additional Scenario

• All items should be selected

Actual behavior

Only Item 3 is selected after the Shift-Click

Additional Scenario:
Only Items 3 to 5 are selected

Regression?

Tested in NetFx 4.81: same issue

Known Workarounds

No response

Impact

No response

Configuration

No response

Other information

_selectionAnchor is only updated on user input and the selection code does not check _selectionAnchor against the current true selection.
DataGrid.cs