Keep dialog glass border on narrow screens (#12591)

* Keep dialog glass border on narrow screens

It got pushed off the edge and eventually disappeared on narrow
screens, so force it to always be present.

* Update screenshot

Author: dbkr

playwright/snapshots/settings/general-user-settings-tab.spec.ts/general-smallscreen-linux.png

@@ -332,7 +332,10 @@ legend {
 .mx_Dialog_border {
     z-index: var(--dialog-zIndex-standard);
     position: relative;
-    max-height: calc(100% - var(--cpd-space-12x));
+    width: 100%;
+    max-width: min-content;
+    box-sizing: border-box;
+    max-height: calc(100% - var(--cpd-space-6x));
     display: flex;
     flex-direction: column;