layout: fix expander appearing bug (tensorflow#5503)

When the sidebar got resized and the width is small enough, we collapsed it. The bug is the expander used to reopen the sidebar disappears and the sidebar can never be expanded again. (What's worse is we remember this setting in local storage). This pr fixes the bug.

tensorboard/webapp/core/views/layout_container.scss

@@ -30,7 +30,6 @@ limitations under the License.
 .expand {
   @include tb-theme-foreground-prop(border-color, border);
   box-sizing: border-box;
-  contain: strict;
   flex: 0 0;
   justify-self: stretch;
 }
@@ -44,6 +43,7 @@ limitations under the License.
   border-style: solid;
   border-width: 0 2px;
   cursor: ew-resize;
+  contain: strict;
   display: flex;
   justify-self: stretch;
 
@@ -69,6 +69,7 @@ limitations under the License.
   border-style: solid;
   border-width: 0 1px 0 0;
   color: inherit;
+  contain: content;
   cursor: pointer;
   display: flex;
   justify-self: stretch;