Bug 1546832 - Adjust letterboxing to take the FindBar and Devtools into account r=johannh

Differential Revision: https://phabricator.services.mozilla.com/D31639

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

browser/actors/RFPHelperChild.jsm

@@ -19,4 +19,9 @@ class RFPHelperChild extends ActorChild {
       this.mm.sendAsyncMessage("Letterboxing:ContentSizeUpdated");
     }
   }
+  receiveMessage(aMessage) {
+    if (isLetterboxingEnabled) {
+      this.mm.sendAsyncMessage("Letterboxing:ContentSizeUpdated");
+    }
+  }
 }

browser/components/BrowserGlue.jsm

@@ -276,6 +276,10 @@ let LEGACY_ACTORS = {
       events: {
         "resize": {},
       },
+      messages: [
+        "Finder:FindbarOpen",
+        "Finder:FindbarClose",
+      ],
     },
   },
 

toolkit/components/resistfingerprinting/RFPHelper.jsm

@@ -370,11 +370,20 @@ class _RFPHelper {
         let containerWidth = browserContainer.clientWidth;
         let containerHeight = browserContainer.clientHeight;
 
+        // If the findbar or devtools are out, we need to subtract their height (plus 1
+        // for the separator) from the container height, because we need to adjust our
+        // letterboxing to acount for it; however it is not included in that dimension
+        // (but rather is subtracted from the content height.)
+        let findBar = browserContainer.getElementsByClassName("findbar-container");
+        let findBarOffset = findBar.length ? findBar[0].clientHeight + 1 : 0;
+        let devtools = browserContainer.getElementsByClassName("devtools-toolbox-bottom-iframe");
+        let devtoolsOffset = devtools.length ? devtools[0].clientHeight + 1 : 0;
+
         return {
           contentWidth,
           contentHeight,
           containerWidth,
-          containerHeight,
+          containerHeight: containerHeight - findBarOffset - devtoolsOffset,
         };
       });