Fix CSS stacking context order determinism

Signed-off-by: Michael Telatynski <7t3chguy@gmail.com>

src/vector/index.html

@@ -108,5 +108,11 @@
     <audio id="remoteAudio"></audio>
     <!-- let CSS themes pass constants to the app -->
     <div id="mx_theme_accentColor"></div><div id="mx_theme_secondaryAccentColor"></div><div id="mx_theme_tertiaryAccentColor"></div>
+
+    <!-- We eagerly create these containers to ensure their CSS stacking context order is sensible -->
+    <div id="mx_PersistedElement_container"></div>
+    <div id="mx_Dialog_StaticContainer"></div>
+    <div id="mx_Dialog_Container"></div>
+    <div id="mx_ContextualMenu_Container"></div>
   </body>
 </html>