A callback that's optionally invoked when a selection handle is tapped.
The TextSelectionControls.buildHandle implementation the text field uses decides where the handle's tap "hotspot" is, or whether the selection handle supports tap gestures at all. For instance, MaterialTextSelectionControls calls onSelectionHandleTapped when the selection handle's "knob" is tapped, while CupertinoTextSelectionControls builds a handle that's not sufficiently large for tapping (as it's not meant to be tapped) so it does not call onSelectionHandleTapped even when tapped.
final VoidCallback? onSelectionHandleTapped;