fix: refine panel appearance and shadow properties for better consistency across platforms

src/browser/base/content/zen-styles/zen-panel-ui.css

@@ -33,7 +33,7 @@ panel[type='arrow'][animate]:not([animate='open']) {
 }
 
 menupopup,
-panel {
+panel[type='arrow'] {
   @media (-moz-windows-mica-popups) {
     appearance: auto !important;
     -moz-default-appearance: menupopup;
@@ -47,4 +47,18 @@ panel {
       animation: none !important;
     }
   }
+
+  @media (-moz-platform: macos) {
+    appearance: auto !important;
+    -moz-default-appearance: menupopup;
+    /* We set the default background here, rather than on ::part(content),
+     * because otherwise it'd interfere with the native look. Non-native-looking
+     * popups should get their background via --panel-background */
+    background-color: Menu;
+    --panel-shadow-margin: 0px !important;
+    --panel-background: transparent !important;
+    --panel-border-color: transparent;
+    /* This should be kept in sync with GetMenuMaskImage() */
+    --panel-border-radius: 6px;
+  }
 }

src/browser/themes/shared/tabbrowser/ctrlTab-css.patch

@@ -1,13 +1,12 @@
 diff --git a/browser/themes/shared/tabbrowser/ctrlTab.css b/browser/themes/shared/tabbrowser/ctrlTab.css
-index 645467864db4a73b7addf3a57fa892beaf9a9614..ad2802872bd0c19ba402d9b9e1b56272ea14818c 100644
+index 645467864db4a73b7addf3a57fa892beaf9a9614..46907c1ac8a10e8aba8ad82530269b660add550d 100644
 --- a/browser/themes/shared/tabbrowser/ctrlTab.css
 +++ b/browser/themes/shared/tabbrowser/ctrlTab.css
-@@ -7,7 +7,7 @@
- /* Ctrl-Tab */
- 
- #ctrlTab-panel {
--  appearance: none;
-+  appearance: none !important;
-   --panel-background: hsla(0,0%,40%,.85);
+@@ -12,6 +12,7 @@
    --panel-color: white;
    --panel-border-color: transparent;
+   --panel-padding: 20px 10px 10px;
++  --panel-shadow-margin: 0;
+ }
+ 
+ @media (-moz-platform: macos) {