Bug 1398390 - Only react to mousedown if it is a left-click. r=Gijs

MozReview-Commit-ID: Ie2Xgasg2eN

browser/components/customizableui/content/panelUI.js

@@ -359,6 +359,9 @@ const PanelUI = {
 
     let domEvent = null;
     if (aEvent) {
+      if (aEvent.type == "mousedown" && aEvent.button != 0) {
+        return;
+      }
       if (aEvent.type == "command" && aEvent.inputSource != null) {
         // Synthesize a new DOM mouse event to pass on the inputSource.
         domEvent = document.createEvent("MouseEvent");