The code that checks whether the cursor was at the end of a row in
Cursor::upDowninText was not able to set boundary correctly. This
causes a hang in because the cursor got stuck on a line and there is an
infinite loop BufferView::dispatch when trying to go down.
The fix is to avoid using the watered-down TextMetrics::x2pos wrapper
around getColumnNearX and use the real thing instead.
Eventually, the last user of x2pos (InsetTabular) should be fixed and
the method should go away.