--- a/browser/devtools/performance/test/head.js
+++ b/browser/devtools/performance/test/head.js
@@ -471,25 +471,25 @@ function waitUntil(predicate, interval =
   return deferred.promise;
 }
 
 // EventUtils just doesn't work!
 
 function dragStart(graph, x, y = 1) {
   x /= window.devicePixelRatio;
   y /= window.devicePixelRatio;
-  graph._onMouseMove({ clientX: x, clientY: y });
-  graph._onMouseDown({ clientX: x, clientY: y });
+  graph._onMouseMove({ testX: x, testY: y });
+  graph._onMouseDown({ testX: x, testY: y });
 }
 
 function dragStop(graph, x, y = 1) {
   x /= window.devicePixelRatio;
   y /= window.devicePixelRatio;
-  graph._onMouseMove({ clientX: x, clientY: y });
-  graph._onMouseUp({ clientX: x, clientY: y });
+  graph._onMouseMove({ testX: x, testY: y });
+  graph._onMouseUp({ testX: x, testY: y });
 }
 
 function dropSelection(graph) {
   graph.dropSelection();
   graph.emit("selecting");
 }
 
 /**

--- a/browser/devtools/profiler/test/browser_profiler_tabbed-browser-03.js
+++ b/browser/devtools/profiler/test/browser_profiler_tabbed-browser-03.js
@@ -71,18 +71,18 @@ registerCleanupFunction(() => {
     "devtools.profiler.ui.show-platform-data", gPrevShowPlatformData);
 });
 
 // EventUtils just doesn't work!
 
 function dragStart(graph, x, y = 1) {
   x /= window.devicePixelRatio;
   y /= window.devicePixelRatio;
-  graph._onMouseMove({ clientX: x, clientY: y });
-  graph._onMouseDown({ clientX: x, clientY: y });
+  graph._onMouseMove({ testX: x, testY: y });
+  graph._onMouseDown({ testX: x, testY: y });
 }
 
 function dragStop(graph, x, y = 1) {
   x /= window.devicePixelRatio;
   y /= window.devicePixelRatio;
-  graph._onMouseMove({ clientX: x, clientY: y });
-  graph._onMouseUp({ clientX: x, clientY: y });
+  graph._onMouseMove({ testX: x, testY: y });
+  graph._onMouseUp({ testX: x, testY: y });
 }

--- a/browser/devtools/shared/test/browser_flame-graph-03a.js
+++ b/browser/devtools/shared/test/browser_flame-graph-03a.js
@@ -86,37 +86,37 @@ function testGraph(graph) {
     "The selection end boundary is correct (8).");
 }
 
 // EventUtils just doesn't work!
 
 function hover(graph, x, y = 1) {
   x /= window.devicePixelRatio;
   y /= window.devicePixelRatio;
-  graph._onMouseMove({ clientX: x, clientY: y });
+  graph._onMouseMove({ testX: x, testY: y });
 }
 
 function dragStart(graph, x, y = 1) {
   x /= window.devicePixelRatio;
   y /= window.devicePixelRatio;
-  graph._onMouseMove({ clientX: x, clientY: y });
-  graph._onMouseDown({ clientX: x, clientY: y });
+  graph._onMouseMove({ testX: x, testY: y });
+  graph._onMouseDown({ testX: x, testY: y });
 }
 
 function dragStop(graph, x, y = 1) {
   x /= window.devicePixelRatio;
   y /= window.devicePixelRatio;
-  graph._onMouseMove({ clientX: x, clientY: y });
-  graph._onMouseUp({ clientX: x, clientY: y });
+  graph._onMouseMove({ testX: x, testY: y });
+  graph._onMouseUp({ testX: x, testY: y });
 }
 
 let HORIZONTAL_AXIS = 1;
 let VERTICAL_AXIS = 2;
 
 function scroll(graph, wheel, axis, x, y = 1) {
   x /= window.devicePixelRatio;
   y /= window.devicePixelRatio;
-  graph._onMouseMove({ clientX: x, clientY: y });
-  graph._onMouseWheel({ clientX: x, clientY: y, axis, detail: wheel, axis,
+  graph._onMouseMove({ testX: x, testY: y });
+  graph._onMouseWheel({ testX: x, testY: y, axis, detail: wheel, axis,
     HORIZONTAL_AXIS,
     VERTICAL_AXIS
   });
 }

--- a/browser/devtools/shared/test/browser_flame-graph-03b.js
+++ b/browser/devtools/shared/test/browser_flame-graph-03b.js
@@ -63,14 +63,14 @@ function testGraph(graph) {
 // EventUtils just doesn't work!
 
 let HORIZONTAL_AXIS = 1;
 let VERTICAL_AXIS = 2;
 
 function scroll(graph, wheel, axis, x, y = 1) {
   x /= window.devicePixelRatio;
   y /= window.devicePixelRatio;
-  graph._onMouseMove({ clientX: x, clientY: y });
-  graph._onMouseWheel({ clientX: x, clientY: y, axis, detail: wheel, axis,
+  graph._onMouseMove({ testX: x, testY: y });
+  graph._onMouseWheel({ testX: x, testY: y, axis, detail: wheel, axis,
     HORIZONTAL_AXIS,
     VERTICAL_AXIS
   });
 }

--- a/browser/devtools/shared/test/browser_flame-graph-03c.js
+++ b/browser/devtools/shared/test/browser_flame-graph-03c.js
@@ -116,24 +116,24 @@ function testGraph(graph) {
     "The vertical offset is correct (9).");
 }
 
 // EventUtils just doesn't work!
 
 function hover(graph, x, y = 1) {
   x /= window.devicePixelRatio;
   y /= window.devicePixelRatio;
-  graph._onMouseMove({ clientX: x, clientY: y });
+  graph._onMouseMove({ testX: x, testY: y });
 }
 
 function dragStart(graph, x, y = 1) {
   x /= window.devicePixelRatio;
   y /= window.devicePixelRatio;
-  graph._onMouseMove({ clientX: x, clientY: y });
-  graph._onMouseDown({ clientX: x, clientY: y });
+  graph._onMouseMove({ testX: x, testY: y });
+  graph._onMouseDown({ testX: x, testY: y });
 }
 
 function dragStop(graph, x, y = 1) {
   x /= window.devicePixelRatio;
   y /= window.devicePixelRatio;
-  graph._onMouseMove({ clientX: x, clientY: y });
-  graph._onMouseUp({ clientX: x, clientY: y });
+  graph._onMouseMove({ testX: x, testY: y });
+  graph._onMouseUp({ testX: x, testY: y });
 }

--- a/browser/devtools/shared/test/browser_graphs-06.js
+++ b/browser/devtools/shared/test/browser_graphs-06.js
@@ -79,12 +79,12 @@ function map(value, istart, istop, ostar
   return ostart + (ostop - ostart) * ((value - istart) / (istop - istart));
 }
 
 // EventUtils just doesn't work!
 
 function click(graph, x, y = 1) {
   x /= window.devicePixelRatio;
   y /= window.devicePixelRatio;
-  graph._onMouseMove({ clientX: x, clientY: y });
-  graph._onMouseDown({ clientX: x, clientY: y });
-  graph._onMouseUp({ clientX: x, clientY: y });
+  graph._onMouseMove({ testX: x, testY: y });
+  graph._onMouseDown({ testX: x, testY: y });
+  graph._onMouseUp({ testX: x, testY: y });
 }

--- a/browser/devtools/shared/test/browser_graphs-07a.js
+++ b/browser/devtools/shared/test/browser_graphs-07a.js
@@ -167,50 +167,50 @@ function testGraph(graph, dragStop) {
     "The current selection end value is correct (17).");
 }
 
 // EventUtils just doesn't work!
 
 function hover(graph, x, y = 1) {
   x /= window.devicePixelRatio;
   y /= window.devicePixelRatio;
-  graph._onMouseMove({ clientX: x, clientY: y });
+  graph._onMouseMove({ testX: x, testY: y });
 }
 
 function click(graph, x, y = 1) {
   x /= window.devicePixelRatio;
   y /= window.devicePixelRatio;
-  graph._onMouseMove({ clientX: x, clientY: y });
-  graph._onMouseDown({ clientX: x, clientY: y });
-  graph._onMouseUp({ clientX: x, clientY: y });
+  graph._onMouseMove({ testX: x, testY: y });
+  graph._onMouseDown({ testX: x, testY: y });
+  graph._onMouseUp({ testX: x, testY: y });
 }
 
 function dragStart(graph, x, y = 1) {
   x /= window.devicePixelRatio;
   y /= window.devicePixelRatio;
-  graph._onMouseMove({ clientX: x, clientY: y });
-  graph._onMouseDown({ clientX: x, clientY: y });
+  graph._onMouseMove({ testX: x, testY: y });
+  graph._onMouseDown({ testX: x, testY: y });
 }
 
 function normalDragStop(graph, x, y = 1) {
   x /= window.devicePixelRatio;
   y /= window.devicePixelRatio;
-  graph._onMouseMove({ clientX: x, clientY: y });
-  graph._onMouseUp({ clientX: x, clientY: y });
+  graph._onMouseMove({ testX: x, testY: y });
+  graph._onMouseUp({ testX: x, testY: y });
 }
 
 function buggyDragStop(graph, x, y = 1) {
   x /= window.devicePixelRatio;
   y /= window.devicePixelRatio;
 
   // Only fire a mousemove instead of a mouseup.
   // This happens when the mouseup happens outside of the toolbox,
   // see Bug 1066504.
-  graph._onMouseMove({ clientX: x, clientY: y });
-  graph._onMouseMove({ clientX: x, clientY: y, buttons: 0 });
+  graph._onMouseMove({ testX: x, testY: y });
+  graph._onMouseMove({ testX: x, testY: y, buttons: 0 });
 }
 
 function scroll(graph, wheel, x, y = 1) {
   x /= window.devicePixelRatio;
   y /= window.devicePixelRatio;
-  graph._onMouseMove({ clientX: x, clientY: y });
-  graph._onMouseWheel({ clientX: x, clientY: y, detail: wheel });
+  graph._onMouseMove({ testX: x, testY: y });
+  graph._onMouseWheel({ testX: x, testY: y, detail: wheel });
 }

--- a/browser/devtools/shared/test/browser_graphs-07b.js
+++ b/browser/devtools/shared/test/browser_graphs-07b.js
@@ -43,24 +43,24 @@ function testGraph(graph) {
     "The graph shouldn't have a selection (3).");
 }
 
 // EventUtils just doesn't work!
 
 function hover(graph, x, y = 1) {
   x /= window.devicePixelRatio;
   y /= window.devicePixelRatio;
-  graph._onMouseMove({ clientX: x, clientY: y });
+  graph._onMouseMove({ testX: x, testY: y });
 }
 
 function dragStart(graph, x, y = 1) {
   x /= window.devicePixelRatio;
   y /= window.devicePixelRatio;
-  graph._onMouseMove({ clientX: x, clientY: y });
-  graph._onMouseDown({ clientX: x, clientY: y });
+  graph._onMouseMove({ testX: x, testY: y });
+  graph._onMouseDown({ testX: x, testY: y });
 }
 
 function dragStop(graph, x, y = 1) {
   x /= window.devicePixelRatio;
   y /= window.devicePixelRatio;
-  graph._onMouseMove({ clientX: x, clientY: y });
-  graph._onMouseUp({ clientX: x, clientY: y });
+  graph._onMouseMove({ testX: x, testY: y });
+  graph._onMouseUp({ testX: x, testY: y });
 }

--- a/browser/devtools/shared/test/browser_graphs-08.js
+++ b/browser/devtools/shared/test/browser_graphs-08.js
@@ -41,26 +41,26 @@ function testGraph(graph) {
     "The selection should be dropped.");
 }
 
 // EventUtils just doesn't work!
 
 function click(graph, x, y = 1) {
   x /= window.devicePixelRatio;
   y /= window.devicePixelRatio;
-  graph._onMouseMove({ clientX: x, clientY: y });
-  graph._onMouseDown({ clientX: x, clientY: y });
-  graph._onMouseUp({ clientX: x, clientY: y });
+  graph._onMouseMove({ testX: x, testY: y });
+  graph._onMouseDown({ testX: x, testY: y });
+  graph._onMouseUp({ testX: x, testY: y });
 }
 
 function dragStart(graph, x, y = 1) {
   x /= window.devicePixelRatio;
   y /= window.devicePixelRatio;
-  graph._onMouseMove({ clientX: x, clientY: y });
-  graph._onMouseDown({ clientX: x, clientY: y });
+  graph._onMouseMove({ testX: x, testY: y });
+  graph._onMouseDown({ testX: x, testY: y });
 }
 
 function dragStop(graph, x, y = 1) {
   x /= window.devicePixelRatio;
   y /= window.devicePixelRatio;
-  graph._onMouseMove({ clientX: x, clientY: y });
-  graph._onMouseUp({ clientX: x, clientY: y });
+  graph._onMouseMove({ testX: x, testY: y });
+  graph._onMouseUp({ testX: x, testY: y });
 }

--- a/browser/devtools/shared/test/browser_graphs-10a.js
+++ b/browser/devtools/shared/test/browser_graphs-10a.js
@@ -116,24 +116,24 @@ function* testGraph(host, graph) {
     "The current selection end value is correct (6).");
 }
 
 // EventUtils just doesn't work!
 
 function hover(graph, x, y = 1) {
   x /= window.devicePixelRatio;
   y /= window.devicePixelRatio;
-  graph._onMouseMove({ clientX: x, clientY: y });
+  graph._onMouseMove({ testX: x, testY: y });
 }
 
 function dragStart(graph, x, y = 1) {
   x /= window.devicePixelRatio;
   y /= window.devicePixelRatio;
-  graph._onMouseMove({ clientX: x, clientY: y });
-  graph._onMouseDown({ clientX: x, clientY: y });
+  graph._onMouseMove({ testX: x, testY: y });
+  graph._onMouseDown({ testX: x, testY: y });
 }
 
 function dragStop(graph, x, y = 1) {
   x /= window.devicePixelRatio;
   y /= window.devicePixelRatio;
-  graph._onMouseMove({ clientX: x, clientY: y });
-  graph._onMouseUp({ clientX: x, clientY: y });
+  graph._onMouseMove({ testX: x, testY: y });
+  graph._onMouseUp({ testX: x, testY: y });
 }

--- a/browser/devtools/shared/test/browser_graphs-12.js
+++ b/browser/devtools/shared/test/browser_graphs-12.js
@@ -130,24 +130,24 @@ function testGraphs(graph1, graph2) {
     "The current selection end value is correct (6.2).");
 }
 
 // EventUtils just doesn't work!
 
 function hover(graph, x, y = 1) {
   x /= window.devicePixelRatio;
   y /= window.devicePixelRatio;
-  graph._onMouseMove({ clientX: x, clientY: y });
+  graph._onMouseMove({ testX: x, testY: y });
 }
 
 function dragStart(graph, x, y = 1) {
   x /= window.devicePixelRatio;
   y /= window.devicePixelRatio;
-  graph._onMouseMove({ clientX: x, clientY: y });
-  graph._onMouseDown({ clientX: x, clientY: y });
+  graph._onMouseMove({ testX: x, testY: y });
+  graph._onMouseDown({ testX: x, testY: y });
 }
 
 function dragStop(graph, x, y = 1) {
   x /= window.devicePixelRatio;
   y /= window.devicePixelRatio;
-  graph._onMouseMove({ clientX: x, clientY: y });
-  graph._onMouseUp({ clientX: x, clientY: y });
+  graph._onMouseMove({ testX: x, testY: y });
+  graph._onMouseUp({ testX: x, testY: y });
 }

--- a/browser/devtools/shared/test/browser_graphs-14.js
+++ b/browser/devtools/shared/test/browser_graphs-14.js
@@ -65,25 +65,25 @@ function* testGraph(graph) {
     "One scroll event should have been fired.");
 }
 
 // EventUtils just doesn't work!
 
 function dragStart(graph, x, y = 1) {
   x /= window.devicePixelRatio;
   y /= window.devicePixelRatio;
-  graph._onMouseMove({ clientX: x, clientY: y });
-  graph._onMouseDown({ clientX: x, clientY: y });
+  graph._onMouseMove({ testX: x, testY: y });
+  graph._onMouseDown({ testX: x, testY: y });
 }
 
 function dragStop(graph, x, y = 1) {
   x /= window.devicePixelRatio;
   y /= window.devicePixelRatio;
-  graph._onMouseMove({ clientX: x, clientY: y });
-  graph._onMouseUp({ clientX: x, clientY: y });
+  graph._onMouseMove({ testX: x, testY: y });
+  graph._onMouseUp({ testX: x, testY: y });
 }
 
 function scroll(graph, wheel, x, y = 1) {
   x /= window.devicePixelRatio;
   y /= window.devicePixelRatio;
-  graph._onMouseMove({ clientX: x, clientY: y });
-  graph._onMouseWheel({ clientX: x, clientY: y, detail: wheel });
+  graph._onMouseMove({ testX: x, testY: y });
+  graph._onMouseWheel({ testX: x, testY: y, detail: wheel });
 }

--- a/browser/devtools/shared/widgets/FlameGraph.js
+++ b/browser/devtools/shared/widgets/FlameGraph.js
@@ -726,19 +726,17 @@ FlameGraph.prototype = {
     }
     return "";
   },
 
   /**
    * Listener for the "mousemove" event on the graph's container.
    */
   _onMouseMove: function(e) {
-    let offset = this._getContainerOffset();
-    let mouseX = (e.clientX - offset.left) * this._pixelRatio;
-    let mouseY = (e.clientY - offset.top) * this._pixelRatio;
+    let {mouseX, mouseY} = this._getRelativeEventCoordinates(e);
 
     let canvasWidth = this._width;
     let canvasHeight = this._height;
 
     let selection = this._selection;
     let selectionWidth = selection.end - selection.start;
     let selectionScale = canvasWidth / selectionWidth;
 
@@ -778,19 +776,17 @@ FlameGraph.prototype = {
       this.emit("panning-vertically");
     }
   },
 
   /**
    * Listener for the "mousedown" event on the graph's container.
    */
   _onMouseDown: function(e) {
-    let offset = this._getContainerOffset();
-    let mouseX = (e.clientX - offset.left) * this._pixelRatio;
-    let mouseY = (e.clientY - offset.top) * this._pixelRatio;
+    let {mouseX, mouseY} = this._getRelativeEventCoordinates(e);
 
     this._selectionDragger.origin = mouseX;
     this._selectionDragger.anchor.start = this._selection.start;
     this._selectionDragger.anchor.end = this._selection.end;
 
     this._verticalOffsetDragger.origin = mouseY;
     this._verticalOffsetDragger.anchor = this._verticalOffset;
 
@@ -812,18 +808,17 @@ FlameGraph.prototype = {
     this._verticalDragDirection = 0;
     this._canvas.removeAttribute("input");
   },
 
   /**
    * Listener for the "wheel" event on the graph's container.
    */
   _onMouseWheel: function(e) {
-    let offset = this._getContainerOffset();
-    let mouseX = (e.clientX - offset.left) * this._pixelRatio;
+    let {mouseX} = this._getRelativeEventCoordinates(e);
 
     let canvasWidth = this._width;
     let canvasHeight = this._height;
 
     let selection = this._selection;
     let selectionWidth = selection.end - selection.start;
     let selectionScale = canvasWidth / selectionWidth;
 
@@ -937,16 +932,37 @@ FlameGraph.prototype = {
       x += node.offsetLeft;
       y += node.offsetTop;
     }
 
     return { left: x, top: y };
   },
 
   /**
+   * Given a MouseEvent, make it relative to this._canvas.
+   * @return object {mouseX,mouseY}
+   */
+  _getRelativeEventCoordinates: function(e) {
+    // For ease of testing, testX and testY can be passed in as the event
+    // object.
+    if ("testX" in e && "testY" in e) {
+      return {
+        mouseX: e.testX * this._pixelRatio,
+        mouseY: e.testY * this._pixelRatio
+      };
+    }
+
+    let offset = this._getContainerOffset();
+    let mouseX = (e.clientX - offset.left) * this._pixelRatio;
+    let mouseY = (e.clientY - offset.top) * this._pixelRatio;
+
+    return {mouseX,mouseY};
+  },
+
+  /**
    * Listener for the "resize" event on the graph's parent node.
    */
   _onResize: function() {
     if (this.hasData()) {
       setNamedTimeout(this._uid, GRAPH_RESIZE_EVENTS_DRAIN, this.refresh);
     }
   }
 };

--- a/browser/devtools/shared/widgets/Graphs.jsm
+++ b/browser/devtools/shared/widgets/Graphs.jsm
@@ -941,20 +941,20 @@ AbstractCanvasGraph.prototype = {
 
   /**
    * Given a MouseEvent, make it relative to this._canvas.
    * @return object {mouseX,mouseY}
    */
   _getRelativeEventCoordinates: function(e) {
     // For ease of testing, testX and testY can be passed in as the event
     // object.  If so, just return this.
-    if (e.screenX === undefined) {
+    if ("testX" in e && "testY" in e) {
       return {
-        mouseX: e.clientX * this._pixelRatio,
-        mouseY: e.clientY * this._pixelRatio
+        mouseX: e.testX * this._pixelRatio,
+        mouseY: e.testY * this._pixelRatio
       };
     }
 
     let quad = this._canvas.getBoxQuads({
       relativeTo: this._topWindow.document
     })[0];
 
     let x = (e.screenX - this._topWindow.screenX) - quad.p1.x;