         
         let isOpen = !(splitView.isSubviewCollapsed(subview))
         
-        var position = (isOpen ? 0 : lastWidth)
+        var position = (isOpen ? 0 : lastExtent)
         if (collapseRightward) {
             position = splitView.frame.size.width - position;
         }

        toggleSubview(self.leftView,
                      dividerIndex: 0,
                      paneView: sourceList.view,
                      lastWidth: &lastSourceWidth,
                      collapseRightward: false,
                      widthConstraint: sourceWidthConstraint)
    }

    fileprivate func toggleSubview(_ subview: NSView,
                                   dividerIndex: Int,
                                   paneView: NSView,
                                   lastExtent: inout CGFloat,
                                   collapseRightward: Bool,
                                   widthConstraint: NSLayoutConstraint?) {
        if splitView.isAnimatingDivider(at: dividerIndex) {
