fix drag animation for react 18

Author: rinick

src/DockDropLayer.tsx

@@ -38,7 +38,7 @@ export class DockDropSquare extends React.PureComponent<DockDropSquareProps, Doc
       targetElement = targetElement.parentElement;
     }
     if (panelData.group === placeHolderStyle && direction !== 'float') {
-      // place holder panel should always have full size drop rect
+      // placeholder panel should always have full size drop rect
       this.context.setDropRect(targetElement, 'middle', this, e);
     } else {
       let dockId = this.context.getDockId();

src/DockLayout.tsx

@@ -411,9 +411,12 @@ export class DockLayout extends DockPortalManager implements DockContext {
     let {dropRect} = this.state;
     if (dropRect) {
       if (direction === 'remove') {
-        if (dropRect.source === source) {
-          this.setState({dropRect: null});
-        }
+        this.setState((oldStates) => {
+          if (oldStates.dropRect.source === source) {
+            return {dropRect: null};
+          }
+          return {};
+        });
         return;
       } else if (dropRect.element === element && dropRect.direction === direction && direction !== 'float') {
         // skip duplicated update except for float dragging