Bug 1599962 - OBT should list remote <browser> elements when debugging non browser windows r=ochameau

Code change suggested by :Gijs to support non browser windows

Differential Revision: https://phabricator.services.mozilla.com/D56349

--HG--
extra : moz-landing-system : lando

devtools/server/actors/root.js

@@ -595,14 +595,28 @@ RootActor.prototype = {
     return { form: processDescriptor.form() };
   },
 
+  /**
+   * Note that this method behaves differently when called for a top level
+   * window.
+   * For a top level window, it will ONLY return remote browser elements.
+   * For any other window, it will return chidren elements, remote or not.
+   *
+   * Also important to note, this method only returns direct children, not the
+   * complete browsing context tree.
+   */
   async _getChildBrowsingContexts(id) {
     // If we have the id of the parent, then we need to get the child
     // contexts in a special way. We have a method on the descriptor
     // to take care of this.
-    const window = Services.wm.getMostRecentWindow(
-      DebuggerServer.chromeWindowType
-    );
-    if (window && window.docShell.browsingContext.id === id) {
+    const parentBrowsingContext = BrowsingContext.get(id);
+    // If this is a parent-process window, and it's top-level (not embedded in a browser),
+    // collect all the remote browsers in the window ourselves. getChildren() will not return
+    // these contexts otherwise.
+    if (
+      parentBrowsingContext.window &&
+      !parentBrowsingContext.embedderElement
+    ) {
+      const { window } = parentBrowsingContext;
       return [
         ...window.document.querySelectorAll(`browser[remote="true"]`),
       ].map(browser => browser.browsingContext);
@@ -610,7 +624,6 @@ RootActor.prototype = {
     // for all other contexts, since we do not need to get contexts of
     // a different type, we can just get the children directly from
     // the BrowsingContext.
-    const parentBrowsingContext = BrowsingContext.get(id);
     return parentBrowsingContext.getChildren();
   },