Revise page to offer large and adaptive viewport.

This involves a few other changes:
- Viewport sticks to the window frame & stays in view.
- Permanent Y-scrollbar on main window, so layout doesn't "jump" when
  it's needed.
- Viewport is now on the left, to separate its scrollbars from those of
  the main window.
- Removed TODO list and put diagnostic junk below the "stack" UI.