expand method
void
expand()
Expand and restore previously collapsed pane.
Implementation
void expand() {
if (!_isLeadingCollapsed && !_isTrailingCollapsed) return;
_isLeadingCollapsed = false;
_isTrailingCollapsed = false;
_ratio = _savedRatio ?? _initialRatio ?? 0.5;
_leadingExtent = null;
notifyListeners();
}