I'd like to be able to click and hold then drag up or down to select emails to select multiple rather than click each one individually. Currently holding and dragging down just triggers a refresh.
Have you tried holding the shift key? Click an email in the list. Hold shift. Click lower in the list. Result all mail between is selected. Now hold the CTRL key and start clicking individual email that are either selected or unselected to change their selection status.
It is not dragging to expand your selection "group" but it is much better than clicking on one message at a time.