Hide search input behind split slider

Ensure search input is covered by the split slider, when it is dragged
all the way to the left similar to how tree nodes is covered.

src/Umbraco.Web.UI.Client/src/less/application/grid.less

@@ -92,17 +92,30 @@ body {
 }
 
 #search-form {
-  display: block;
-  margin: 0px;
-  z-index: 100;
-  position: absolute;
-  top: 0;
-  left: 0;
-  right: 0;
-}
+    display: block;
+    margin: 0;
+    z-index: 100;
+    position: absolute;
+    top: 0;
+    left: 0;
+    right: 0;
+    overflow: hidden;
 
-#search-form form{
-  margin-top: 17px;
+    .form-search {
+        display: -ms-flexbox;
+        display: -webkit-box;
+        display: -webkit-flex;
+        display: flex;
+        margin-top: 17px;
+
+        .umb-search-field {
+            width: auto;
+            min-width: 160px;
+            -webkit-flex: 1;
+            -ms-flex: 1;
+            flex: 1;
+        }
+    }
 }
 
 #navigation {