Bug 1872325 - Reduce macOS menupopup border radius. r=emilio,desktop-…

…theme-reviewers

The macOS menupopup corner radius was recently changed from 4px to 8px, but that does not quite match the system, which measures at 6px.

Differential Revision: https://phabricator.services.mozilla.com/D197408

toolkit/themes/shared/popup.css

@@ -53,7 +53,7 @@ panel {
     --panel-background: none;
     --panel-border-color: transparent;
     /* This should be kept in sync with GetMenuMaskImage() */
-    --panel-border-radius: 8px;
+    --panel-border-radius: 6px;
   }
 
   &::part(content) {

widget/cocoa/nsCocoaWindow.mm

@@ -3592,7 +3592,7 @@ - (id)initWithContentRect:(NSRect)aContentRect
 
 // Returns an autoreleased NSImage.
 static NSImage* GetMenuMaskImage() {
-  const CGFloat radius = 8.0f;
+  const CGFloat radius = 6.0f;
   const NSSize maskSize = {radius * 3.0f, radius * 3.0f};
   NSImage* maskImage = [NSImage imageWithSize:maskSize
                                       flipped:FALSE