Bug 1767787 - [devtools] Wait until default editor has focus in browser_styleeditor_bug_851132_middle_click.js r=nchevobbe

I could reproduce the intermittent locally and when it occurs, the initial editor actually didn't have focus.
I think that what happens is that we hit a race condition where the focus is stolen by the initial editor after we clicked on a new stylesheet
and switched to a new editor.

Differential Revision: https://phabricator.services.mozilla.com/D149952

devtools/client/styleeditor/test/browser_styleeditor_bug_851132_middle_click.js

@@ -14,6 +14,11 @@ add_task(async function() {
   await ui.editors[0].getSourceEditor();
   info("first editor selected");
 
+  await waitFor(
+    () => ui.editors[0].sourceEditor.hasFocus(),
+    "Wait until the initially selected editor grabs the focus"
+  );
+
   info("Left-clicking on the second editor link.");
   await clickOnStyleSheetLink(ui.editors[1], 0);