workaround spurious CodeMirror scrollbars

always draw scrollbar margin, so that inappropriate scrollbars never overlap the last line. To compensate for the extra space, the margin between the scrollbar and the last line is shrunk significantly.

This is full of compromises, so we may well decide not to merge it as-is, but it does seem to mitigate the fairly catastrophic scrollbar issue when text is zoomed.

After investigation, this appears to actually be a bug in Chrome, and it's possible that CM will introduce [its own workaround](https://github.com/marijnh/CodeMirror/pull/1545) in a future release.