Bug 1559317 - prevent on mouse down default action on TreeView's tbody so it never gets focused state. r=mtigley

Differential Revision: https://phabricator.services.mozilla.com/D35335

devtools/client/shared/components/tree/TreeView.js

@@ -673,6 +673,8 @@ define(function(require, exports, module) {
           dom.tbody({
             role: "presentation",
             tabIndex: -1,
+            // Prevent mouse down defaul action so that tbody never gets focused.
+            onMouseDown: event => event.preventDefault(),
           }, rows)
         )
       );