Bug 1584346 - Make RDM UI's getViewportBrowser return the browser tab if fission-enabled r=mconley,bradwerth This revision modifies RDM UI’s `getViewportBrowser` to return the tab’s <browser> instead of the <mozbrowser> this.toolWindow’s [getViewportBrowser method](https://searchfox.org/mozilla-central/source/devtools/client/responsive/index.js#176) returns. If the pref `devtools.responsive.browserUI.enabled` is set to true, this will cause this.toolWindow to be null because this object is set during the [swap step when RDM is initialized](https://searchfox.org/mozilla-central/source/devtools/client/responsive/ui.js#150). Since we're not doing away with swap/tunnel until work on RDM fission is complete, we should still preserve the toolWindow property on RDM UI. For context, this.toolWindow is a reference to the RDM UI’s window, which also happens to be populated with other properties such as `getViewportBrowser` (the function causing the issue), `getViewportSize`, `addInitialViewport`, etc… It’s responsible for holding the RDM UI chrome:// document which is also where the <mozbrowser> iframe is contained. Please see: https://searchfox.org/mozilla-central/source/devtools/client/responsive/index.js for reference on where these properties are added. Also, now that the RDM fission work requires the removal of <mozbrowser>, this.toolWindow won't be needed to get access to the toolbar UI. The RDM toolbar will be embedded into the browser UI and can be referenced with `this.rdmFrame`. So this means toolWindow along with any parts of the code using it should be removed when fission work is complete. I believe [Bug 1585096](https://bugzilla.mozilla.org/show_bug.cgi?id=1585096) would be the correct place to do this. Differential Revision: https://phabricator.services.mozilla.com/D56602

--- a/devtools/client/responsive/ui.js
+++ b/devtools/client/responsive/ui.js
@@ -88,16 +88,18 @@ class ResponsiveUI {
     // Flag set when destruction has ended.
     this.destroyed = false;
     /**
      * A window reference for the chrome:// document that displays the responsive
      * design tool.  It is safe to reference this window directly even with e10s,
      * as the tool UI is always loaded in the parent process.  The web content
      * contained *within* the tool UI on the other hand is loaded in the child
      * process.
+     *
+     * TODO: we should remove this as part of Bug 1585096
      */
     this.toolWindow = null;
     // The iframe containing the RDM UI.
     this.rdmFrame = null;
 
     // Promise resovled when the UI init has completed.
     this.inited = this.init();
 
@@ -839,17 +841,20 @@ class ResponsiveUI {
     await this.inited;
     this.toolWindow.setViewportSize(size);
   }
 
   /**
    * Helper for tests/reloading the viewport. Assumes a single viewport for now.
    */
   getViewportBrowser() {
-    return this.toolWindow.getViewportBrowser();
+    if (!this.isBrowserUIEnabled) {
+      return this.toolWindow.getViewportBrowser();
+    }
+    return this.tab.linkedBrowser;
   }
 
   /**
    * Helper for contacting the viewport content. Assumes a single viewport for now.
    */
   getViewportMessageManager() {
     return this.getViewportBrowser().messageManager;
   }