feat: Make sure XUL store persists compact mode after change, b=(no-bug), c=compact-mode

src/zen/compact-mode/ZenCompactMode.mjs

@@ -91,6 +91,10 @@ var gZenCompactModeManager = {
       this.preference === value ||
       document.documentElement.hasAttribute('zen-compact-animating')
     ) {
+      if (typeof this._wasInCompactMode !== 'undefined') {
+        // We wont do anything with it anyway, so we remove it
+        delete this._wasInCompactMode;
+      }
       // We dont want the user to be able to spam the button
       return value;
     }
@@ -99,6 +103,7 @@ var gZenCompactModeManager = {
     // main-window can't store attributes other than window sizes, so we use this instead
     lazyCompactMode.mainAppWrapper.setAttribute('zen-compact-mode', value);
     document.documentElement.setAttribute('zen-compact-mode', value);
+    Services.xulStore.persist(lazyCompactMode.mainAppWrapper, 'zen-compact-mode');
     this._updateEvent();
     return value;
   },

src/zen/compact-mode/zen-compact-mode.css

@@ -152,6 +152,7 @@
           border-radius: calc(var(--zen-native-inner-radius) + var(--zen-element-separation) / 4);
           @media -moz-pref('zen.view.compact.color-sidebar') {
             background: var(--zen-main-browser-background-toolbar) !important;
+            background-attachment: fixed !important;
             background-size: 2000px !important;
             @media -moz-pref('zen.theme.acrylic-elements') {
               backdrop-filter: blur(42px) saturate(110%) brightness(0.25) contrast(100%) !important;