Flush mouseMove when initiating viewport dragging

We want to flush pending mouse moves before we initiate viewport
dragging.

Before this commit, there were scenarios where the _mouseButtonMask
would track a released button as being down.

core/rfb.js

@@ -1103,6 +1103,8 @@ export default class RFB extends EventTargetMixin {
                         this._viewportDragPos = {'x': pos.x, 'y': pos.y};
                         this._viewportHasMoved = false;
 
+                        this._flushMouseMoveTimer(pos.x, pos.y);
+
                         // Skip sending mouse events, instead save the current
                         // mouse mask so we can send it later.
                         this._mouseButtonMask = bmask;

tests/test.rfb.js

@@ -898,6 +898,24 @@ describe('Remote Frame Buffer protocol client', function () {
 
                 expect(client._display.viewportChangePos).to.not.have.been.called;
             });
+
+            it('should flush move events when initiating viewport drag', function () {
+                sendMouseMoveEvent(13, 9, 0x0, client);
+                sendMouseMoveEvent(14, 9, 0x0, client);
+                sendMouseButtonEvent(14, 9, true, 0x1, client);
+
+                expect(RFB.messages.pointerEvent).to.have.been.calledTwice;
+                expect(RFB.messages.pointerEvent.firstCall).to.have.been.calledWith(client._sock,
+                                                                                    13, 9, 0x0);
+                expect(RFB.messages.pointerEvent.secondCall).to.have.been.calledWith(client._sock,
+                                                                                     14, 9, 0x0);
+
+                RFB.messages.pointerEvent.resetHistory();
+
+                clock.tick(100);
+
+                expect(RFB.messages.pointerEvent).to.not.have.been.called;;
+            });
         });
     });