use display: -webkit-box instead of flex for nav-bar

display: flex breaks extension menus but -webkit-box doesn't, so lets
use that while it works.

chrome/multi-row_tabs_below_content.css

@@ -4,9 +4,6 @@ See the above repository for updates as well as full license text. */
 /* Makes tabs to appear on multiple lines below the web content */
 /* Tab reordering will not work and can't be made to work */
 
-/* !!!WARNING!!! */
-/* This style completely breaks extension popups. As a workaround, you can pin the extension buttons to overflow menu which allows them to work. */
-
 /* Make menubar enabled for getting window controls in correct place. Menubar options will be shown as overlay when mouse is at top-left corner */
 
 /* It's recommended to move tabs new-tab-button outside tabs toolbar */
@@ -41,7 +38,9 @@ See the above repository for updates as well as full license text. */
 
   #nav-bar{
     position: fixed !important;
-    display: flex;
+    /* For some reason -webkit-box behaves internally like -moz-box, but can be used with fixed position. display: flex would work too but it breaks extension menus. */
+    display: -webkit-box;
+    -webkit-box-flex: 1;
     margin-left: 30px;
     width: calc(100vw - 30px);
     top: var(--multirow-top-padding);