extent property Null safety
The position at which the selection terminates.
When the user uses the arrow keys to adjust the selection, this is the value that changes. Similarly, if the current theme paints a caret on one side of the selection, this is the location at which to paint the caret.
Might be larger than, smaller than, or equal to base.
TextPosition get extent => TextPosition(offset: extentOffset, affinity: affinity);