Firefox fix

[texodus]

Rebased and amended version of #36.

• The main fix alters regular-table virtual position calculations to use clientWidth and clientHeight over offsetWidth, offsetHeight, where appropriate, to handle non-zero-width scrollbars.
• Removes style code which was hard to material theme.
• Hardcodes non-overlay scrollbars to the material theme, since it is more important this theme be
  consistent.

[texodus]

Fixes #26