Bug 1788273 - Tweak popup alignment so that it looks good on mirrored wayland popups. r=dao,mak

When these panels had arrows, I guess the bottomcenter topleft alignment
made sense so that you could precisely align the arrow, but that's not
what we do now.

Don't use bottomcenter / leftcenter / rightcenter, since we really want
the sides to align.

This shouldn't change behavior on any platform except Linux + Wayland,
where the alignment looks good now in the case of bug 1784876.

Differential Revision: https://phabricator.services.mozilla.com/D156099

browser/components/customizableui/content/panelUI.js

@@ -489,7 +489,7 @@ const PanelUI = {
 
       try {
         viewShown = await PanelMultiView.openPopup(tempPanel, anchor, {
-          position: "bottomcenter topright",
+          position: "bottomright topright",
           triggerEvent: aEvent,
         });
       } catch (ex) {
@@ -855,7 +855,7 @@ const PanelUI = {
         el.removeAttribute("data-lazy-l10n-id");
       });
 
-    this.notificationPanel.openPopup(anchor, "bottomcenter topright");
+    this.notificationPanel.openPopup(anchor, "bottomright topright");
   },
 
   _clearNotificationPanel() {