The last visible range can extend past the visible column on text
nodes that span multiple columns (its end-offset bisect uses union
rects), so comparing DOM boundary points against it misses off-view
selection endpoints. Test the endpoint caret rect against the
visible column bounds instead.
Chrome emits collapsed selectionchange events while the long-press
word selection is being set up; clearing the flag on them disarmed
the clamp before the selection ever became a Range.
Chrome word-snapping during a touch selection drag can map the
endpoint past the visible column into the next (off-screen) page,
selecting the entire visible page at once. Re-map the endpoint via
caretRangeFromPoint at the same line height inside the visible
column whenever it lands outside the last visible range.
John O'Keefe