multirow tabs: apply flex:1 to scrollbox

chrome/multi-row_tabs.css

@@ -29,6 +29,7 @@ See the above repository for updates as well as full license text. */
   scrollbox[part][orient="horizontal"] > slot,
   scrollbox[part][orient="horizontal"]{
     display: flex;
+    flex: 1;
     flex-wrap: wrap;
     overflow-y: auto;
     max-height: calc((var(--tab-min-height) + 2 * var(--tab-block-margin,0px)) * var(--multirow-n-rows));

chrome/multi-row_tabs_below_content.css

@@ -180,6 +180,7 @@ See the above repository for updates as well as full license text. */
   scrollbox[part][orient="horizontal"] > slot,
   scrollbox[part][orient="horizontal"]{
     display: flex;
+    flex: 1;
     flex-wrap: wrap;
     overflow-y: auto;
     max-height: var(--uc-scrollbox-max-height,1px);