Filtering out transform actions is indeed the recommended approach to making an editor read-only. But since 0.15 you’ll still see edits flash and then disappear when you make them, which is non-optimal. Yet turning off contentEditable breaks selection, as you noted.
We can go in two directions, depending on whether we want to have a caret in readOnly mode. Either change the way events are handled to suppress DOM editing when readOnly is enabled, or disable contenteditable entirely, and handle the correspondence between the selection state and the DOM selection differently in that case.
I’m leaning towards the second. Does anyone feel having a cursor when the content isn’t editable is a useful feature?