Backed out changeset 34d5cbb640e3 (bug 1530557) for failing bc at browser_double_close_tab.js

toolkit/components/prompts/content/tabprompts.jsm

@@ -245,8 +245,8 @@ var TabModalPrompt = class {
   }
 
   onResize() {
-    let availWidth = this.element.clientWidth;
-    let availHeight = this.element.clientHeight;
+    let availWidth = this.clientWidth;
+    let availHeight = this.clientHeight;
     if (availWidth == this.availWidth && availHeight == this.availHeight)
       return;
     this.availWidth = availWidth;