--- a/devtools/client/inspector/inspector.js
+++ b/devtools/client/inspector/inspector.js
@@ -521,18 +521,21 @@ Inspector.prototype = {
     this.sidebar.off("destroy", this.onSidebarHidden);
   },
 
   /**
    * If Toolbox width is less than 600 px, the splitter changes its mode
    * to `horizontal` to support portrait view.
    */
   onPanelWindowResize: function() {
-    this.splitBox.setState({
-      vert: this.useLandscapeMode(),
+    this.panelWin.cancelIdleCallback(this._resizeTimerId);
+    this._resizeTimerId = this.panelWin.requestIdleCallback(() => {
+      this.splitBox.setState({
+        vert: this.useLandscapeMode(),
+      });
     });
   },
 
   getSidebarSize: function() {
     let width;
     let height;
     let splitSidebarWidth;