previewAnchoredAtStart property
A preview highlights a range with two endpoints - one is preview,
and the other is either the start (if previewAnchoredAtStart is true) or
the end (if previewAnchoredAtStart is false) of the currentSelection.
This variable is still relevant even if preview is null, because it
specifies which endpoint will become the anchor if previewing begins.
Implementation
final bool previewAnchoredAtStart;