fix: various popups flickering (MR 25)

The protection, identity, permissions and widget overflow popups were
flickering when their width was larger than the screen. This patch
bounds the width of the viewport.
It also makes the overflow menu use as much vertical real estate as
possible, as this seems to fix flickers for extensions.

src/userChrome/popups.css

@@ -30,6 +30,7 @@
         margin-top: -250px;
         height: 200px;
         min-height: 200px;
+        max-width: 100vw !important;
     }
 
     /* Hide some context menu items */
@@ -42,4 +43,29 @@
     #inspect-separator {
         display: none !important
     }
+
+    /* fix flickering of the protections, permissions,
+     * widget overflow and identity popups */
+    #protections-popup,
+    #permission-popup,
+    #widget-overflow,
+    #identity-popup {
+        max-width: 100vw !important;
+    }
+
+    /* fix the protections popup gettting
+     * too wide, making controls unaccessible */
+    #protections-popup-mainView {
+        min-width: 100vw !important;
+        max-width: 100vw !important;
+    }
+
+    /* fix flicker on extension menus.
+     * The compromise is that the overflow menu always use
+     * all available height.
+     * The -80px is here to prevent covering the main bars */
+    #widget-overflow,
+    #widget-overflow-mainView {
+        height: calc(100vh - 80px) !important;
+    }
 }