Fix firefox display of sound editor by making sure chain is all flex

src/components/asset-panel/asset-panel.css

@@ -12,7 +12,8 @@
 }
 
 .detail-area {
+    display: flex;
     flex-grow: 1;
-    flex-shrink: 0;
+    flex-shrink: 1;
     border-left: 1px solid $ui-pane-border;
 }

src/components/sound-editor/sound-editor.css

@@ -4,6 +4,7 @@
 .editor-container {
     display: flex;
     flex-direction: column;
+    flex-grow: 1;
     padding: calc(2 * $space);
 }