Steps to reproduce

1.  Open Pages app
2.  Select some items
3. De-select some items
4. Select some more items
5. Previously selected items are de-selected randomly.

I have a video showing this but it exceeds the 10MB limit for attachments.

Expected results

 All selected items should remain selected.

Actual results

Some selected items are randomly deselected

Workaround

Be very careful when clicking checkboxes to not click on the row area around.
 

Development notes

Left click could be active only in column area (click into the zero column is ignored instead of triggering single selection)