Comment 201 for bug 584632

Created attachment 8584346
Unified patch (code + test changes + twice revised new test)

Thank you very much for the review and all the additional explanation.
I hope I could address your questions in additional comments in the test.

Sadly switching from "mousedown/up" to "click" as you had suggested makes the test fail with:
failed | uncaught exception - NS_ERROR_FAILURE: Component returned failure code: 0x80004005 (NS_ERROR_FAILURE) [nsIDOMWindowUtils.sendMouseEvent] at chrome://specialpowers/content/specialpowersAPI.js:161

This is actually the only test I intended to submit ;-)

This is for the case where the user clicks beyond the end of the line.
There are two tests already which I had to adapt to the new selection behaviour which test the moving to the end
(editor/libeditor/tests/test_selection_move_commands.xul)
and moving from the subsequent line using left arrow
(layout/generic/test/test_movement_by_characters.html).

If you wanted to be really purist a test with a "div contenteditable" could be added where an empty line is first filled with some text, which is then removed so the range becomes empty. The code caters for this case.