Fix the image view (#12341)

res/css/_common.pcss

@@ -335,6 +335,12 @@ legend {
     max-height: calc(100% - var(--cpd-space-12x));
     display: flex;
     flex-direction: column;
+
+    .mx_Dialog_lightbox & {
+        /* The lightbox isn't so much of a dialog as a fullscreen overlay. We
+        don't want the glass border. */
+        display: contents;
+    }
 }
 
 .mx_Dialog {