R1:bf43d4cf2aa6

Don't mutate DOM on touch-originated cursor events in Differential

Summary:
Fixes T10229. Broadly:

- When the user hovers over a line number or inline comment, we update the yellow reticle to highlight the relevant lines. Specifically, this is in response to a `mouseover` event.
- On touch devices, touches fire `mouseover` and if you mutate the DOM inside the event, the device aborts the touch.

To remedy this:

- Distingiush between mouse-originated and touch-originated cursor events.
- We do…