Bug 1561227 Part 3: Make RDM UI save and restore resolution when responding to a full zoom change. r=mtigley

This change makes the RDM content frame script listen to the new
PreFullZoomChange event, and treat that as a trigger to save the
existing resolution. The content window will send 2 resize events
as it adjusts to the new RDM pane size set by the front end. After
these events are received, the resolution is restored and a new
event is fired that indicates the work of zooming is complete.

Differential Revision: https://phabricator.services.mozilla.com/D48624

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

devtools/client/responsive/browser/content.js

@@ -28,6 +28,9 @@ var global = this;
   addMessageListener("ResponsiveMode:Stop", stopResponsiveMode);
   addMessageListener("ResponsiveMode:IsActive", isActive);
 
+  // Track the resolution and re-apply it after a full zoom change.
+  let resolutionBeforeFullZoom = 0;
+
   function debug(msg) {
     // dump(`RDM CHILD: ${msg}\n`);
   }
@@ -60,6 +63,16 @@ var global = this;
       startOnResize();
     }
 
+    addEventListener("PreFullZoomChange", onPreFullZoomChange);
+    addEventListener(
+      "mozupdatedremoteframedimensions",
+      onUpdatedRemoteFrameDimensions,
+      {
+        capture: true,
+        mozSystemGroup: true,
+      }
+    );
+
     // At this point, a content viewer might not be loaded for this
     // docshell. setDocumentInRDMPane and makeScrollbarsFloating will be
     // triggered by onLocationChange.
@@ -71,6 +84,20 @@ var global = this;
     sendAsyncMessage("ResponsiveMode:Start:Done");
   }
 
+  function onPreFullZoomChange(event) {
+    if (event.originalTarget == content.document) {
+      resolutionBeforeFullZoom = content.windowUtils.getResolution();
+    }
+  }
+
+  function onUpdatedRemoteFrameDimensions(event) {
+    if (event.originalTarget == content.document) {
+      content.windowUtils.setResolutionAndScaleTo(resolutionBeforeFullZoom);
+      const e = new CustomEvent("ZoomComplete", { bubbles: true });
+      content.dispatchEvent(e);
+    }
+  }
+
   function onResize() {
     // Send both a content-resize event and a viewport-resize event, since both
     // may have changed.
@@ -136,6 +163,11 @@ var global = this;
     restoreScrollbars();
     setDocumentInRDMPane(false);
     stopOnResize();
+    removeEventListener("PreFullZoomChange", onPreFullZoomChange);
+    removeEventListener(
+      "mozupdatedremoteframedimensions",
+      onUpdatedRemoteFrameDimensions
+    );
     sendAsyncMessage("ResponsiveMode:Stop:Done");
   }