1. Click on the submission to start annotating an assignment.
2. Right-click on the page image to bring up the context menu - it is offset slightly from where it appears in FF/Chrome.
3. Select an item from the quicklist - it is inserted to the left and below the original right-click (unlike with FF/Chrome, which position it correctly)

Note: left-clicking to insert a comment puts it in exactly the right place, so I need to work out how these two inserts differ.