Bug 1354782 - Re-use pinned tab width to avoid layout flush and invalidation loop in _positionPinnedTabs. r=florian

MozReview-Commit-ID: 98eGdDUXLm2

--HG--
extra : rebase_source : d454599f50aabbefd6144d7051cb293025e7e1ae

browser/base/content/tabbrowser.xml

@@ -6075,16 +6075,19 @@
 
             let scrollButtonWidth = this.mTabstrip._scrollButtonDown.getBoundingClientRect().width;
             let paddingStart = this.mTabstrip.scrollboxPaddingStart;
+            let pinnedTabWidth;
             let width = 0;
 
             for (let i = numPinned - 1; i >= 0; i--) {
               let tab = this.childNodes[i];
-              width += tab.getBoundingClientRect().width;
+              if (!pinnedTabWidth) {
+                pinnedTabWidth = tab.getBoundingClientRect().width;
+              }
+              width += pinnedTabWidth;
               tab.style.marginInlineStart = -(width + scrollButtonWidth + paddingStart) + "px";
             }
 
             this.style.paddingInlineStart = width + paddingStart + "px";
-
           } else {
             this.removeAttribute("positionpinnedtabs");