Keep the selection of the text field when changed by JS.

Source/core/input/EventHandler.cpp

Index: Source/core/input/EventHandler.cpp
diff --git a/Source/core/input/EventHandler.cpp b/Source/core/input/EventHandler.cpp
index 9e1072c47e5fbfb6e410e44f5368e2cf7c25f900..39bcfff68e989a25bd1278d9d6a646bc206ec421 100644
--- a/Source/core/input/EventHandler.cpp
+++ b/Source/core/input/EventHandler.cpp
@@ -368,7 +368,6 @@ bool EventHandler::handleMousePressEvent(const MouseEventWithHitTestResults& eve
 
     bool swallowEvent = false;
     m_mousePressed = true;
-    selectionController().initializeSelectionState();
 
     if (event.event().clickCount() == 2)
         swallowEvent = selectionController().handleMousePressEventDoubleClick(event);
@@ -956,6 +955,12 @@ bool EventHandler::handleMousePressEvent(const PlatformMouseEvent& mouseEvent)
     m_frame->selection().setCaretBlinkingSuspended(true);
 
     bool swallowEvent = !dispatchMouseEvent(EventTypeNames::mousedown, mev.innerNode(), m_clickCount, mouseEvent, true);
+    // m_selectionInitiationState is initialized after dispatching mousedown
+    // event in order not to keep the selection by DOM APIs Because we can't
+    // give the user the chance to handle the selection by user action like
+    // dragging if we keep the selection in case of mousedown. FireFox also has
+    // the same behavior and it's more compatible with other browsers.
+    selectionController().initializeSelectionState();
     HitTestResult hitTestResult = hitTestResultInFrame(m_frame, mouseEvent.position(), HitTestRequest::ReadOnly);
     swallowEvent = swallowEvent || handleMouseFocus(MouseEventWithHitTestResults(mouseEvent, hitTestResult));
     m_capturesDragging = !swallowEvent || mev.scrollbar();
@@ -2032,6 +2037,7 @@ bool EventHandler::handleGestureTap(const GestureEventWithHitTestResults& target
         static_cast<PlatformEvent::Modifiers>(modifiers | PlatformEvent::LeftButtonDown),
         PlatformMouseEvent::FromTouch,  gestureEvent.timestamp());
     bool swallowMouseDownEvent = !dispatchMouseEvent(EventTypeNames::mousedown, currentHitTest.innerNode(), gestureEvent.tapCount(), fakeMouseDown, true);
+    selectionController().initializeSelectionState();
     if (!swallowMouseDownEvent)
         swallowMouseDownEvent = handleMouseFocus(MouseEventWithHitTestResults(fakeMouseDown, currentHitTest));
     if (!swallowMouseDownEvent)