[VirtualFlow] multiple selection mode : ctrl+shift+space clears selection

According to spec :

Ctrl-Shift-Space Select all items between anchor and focused item. Focused item is also included in the selection. Available in multi-selection mode only.

and according to behavior on win7,

ctrl+shift+space should not clear previous selection, but it does.

Suppose, you have items 1, 2, 3, ... in the list.

Click on 2.
ctrl+down.
ctrl+shift+space.
you have 2,3 selected.
ctrl+up * 2 times.
focus on 1.
ctrl+shift+space.
1,2 are selected. But I expect 1, 2, 3 to be selected