Bug 1511112 - Remove remaining style flushes for getting the UI direction in browser/, except PlacesUIUtils.jsm. r=florian

Differential Revision: https://phabricator.services.mozilla.com/D13536

--HG--
extra : moz-landing-system : lando

browser/base/content/urlbarBindings.xml

@@ -2122,13 +2122,7 @@ file, You can obtain one at http://mozilla.org/MPL/2.0/.
           }
 
           // Set the direction of the popup based on the textbox (bug 649840).
-          // getComputedStyle causes a layout flush, so avoid calling it if a
-          // direction has already been set.
-          if (!this.style.direction) {
-            this.style.direction =
-              aElement.ownerGlobal.getComputedStyle(aElement).direction;
-          }
-          let popupDirection = this.style.direction;
+          let popupDirection = this.style.direction = (RTL_UI ? "rtl" : "ltr");
 
           // Make the popup span the width of the window.  First, set its width.
           let documentRect =

browser/components/newtab/lib/CFRPageActions.jsm

@@ -258,9 +258,7 @@ class PageAction {
 
     headerLabel.value = await this.getStrings(content.heading_text);
     headerLink.setAttribute("href", SUMO_BASE_URL + content.info_icon.sumo_path);
-    const isRTL = this.window.getComputedStyle(notification).direction === "rtl";
-    const attribute = isRTL ? "left" : "right";
-    headerLink.setAttribute(attribute, 0);
+    headerLink.setAttribute(this.window.RTL_UI ? "left" : "right", 0);
     headerImage.setAttribute("tooltiptext", await this.getStrings(content.info_icon.label, "tooltiptext"));
     headerLink.onclick = () => this._sendTelemetry({message_id: id, bucket_id: content.bucket_id, event: "RATIONALE"});
 

browser/components/places/content/menu.xml

@@ -548,7 +548,7 @@
             arrowbox.style.transform = "translate(0, " + -offset + "px)";
 
             // The assigned side stays the same regardless of direction.
-            var isRTL = (window.getComputedStyle(this).direction == "rtl");
+            let isRTL = this.matches(":-moz-locale-dir(rtl)");
 
             if (position.indexOf("start_") == 0) {
               container.dir = "reverse";