Don't force layout for mouse event hit tests

Source/core/dom/Document.cpp

Index: Source/core/dom/Document.cpp
diff --git a/Source/core/dom/Document.cpp b/Source/core/dom/Document.cpp
index 2805c26655f0d17f1a842b09222087e09353068f..cff2dc256274e675a021d08777efb73a00938c43 100644
--- a/Source/core/dom/Document.cpp
+++ b/Source/core/dom/Document.cpp
@@ -2796,7 +2796,13 @@ MouseEventWithHitTestResults Document::prepareMouseEvent(const HitTestRequest& r
 {
     ASSERT(!renderer() || renderer()->isRenderView());
 
-    if (!renderer())
+    // RenderView::hitTest causes a layout, and we don't want to hit that until the first
+    // layout because until then, there is nothing shown on the screen - the user can't
+    // have intentionally clicked on something belonging to this page. Furthermore,
+    // mousemove events before the first layout should not lead to a premature layout()
+    // happening, which could show a flash of white.
+    // See also the similar code in EventHandler::hitTestResultAtPoint.
+    if (!renderer() || !view() || !view()->didFirstLayout())
         return MouseEventWithHitTestResults(event, HitTestResult(LayoutPoint()));
 
     HitTestResult result(documentPoint);