Refine scrollbar interactions

The previous editor had a host of scrollbar-related issues that are addressed in this PR:

* The horizontal scrollbar would obscure the last line.
* The vertical scrollbar would obscure the last character.
* The horizontal scrollbar didn't extend the entire width of the editor.
* The dummy scrollbars were always 15px wide rather than the true width of a scrollbar based on the stylesheet.
* The dummy scrollbars remained visible even when content was not scrollable, interfering with mouse clicks.
* Scrollbars didn't respond to stylesheet changes until they were hidden and redisplayed.