Update viewport positions in two stages

src/intro_gui.cpp

@@ -251,7 +251,8 @@ struct SelectGameWindow : public Window {
 		/* Update the viewport position. */
 		mw->viewport->dest_scrollpos_x = mw->viewport->scrollpos_x = pos.x;
 		mw->viewport->dest_scrollpos_y = mw->viewport->scrollpos_y = pos.y;
-		UpdateViewportPosition(mw);
+		UpdateNextViewportPosition(mw);
+		ApplyNextViewportPosition(mw);
 		mw->SetDirty(); // Required during panning, otherwise logo graphics disappears
 
 		/* If there is only one command, we just executed it and don't need to do any more */

src/viewport.cpp

@@ -4023,10 +4023,10 @@ static inline void ClampViewportToMap(const Viewport *vp, int *scroll_x, int *sc
 }
 
 /**
- * Update the viewport position being displayed.
+ * Update the next viewport position being displayed.
  * @param w %Window owning the viewport.
  */
-void UpdateViewportPosition(Window *w)
+void UpdateNextViewportPosition(Window *w)
 {
 	const Viewport *vp = w->viewport;
 
@@ -4034,9 +4034,9 @@ void UpdateViewportPosition(Window *w)
 		const Vehicle *veh = Vehicle::Get(w->viewport->follow_vehicle);
 		Point pt = MapXYZToViewport(vp, veh->x_pos, veh->y_pos, veh->z_pos);
 
-		w->viewport->scrollpos_x = pt.x;
-		w->viewport->scrollpos_y = pt.y;
-		SetViewportPosition(w, pt.x, pt.y, false);
+		w->viewport->next_scrollpos_x = pt.x;
+		w->viewport->next_scrollpos_y = pt.y;
+		w->viewport->force_update_overlay_pending = false;
 	} else {
 		/* Ensure the destination location is within the map */
 		ClampViewportToMap(vp, &w->viewport->dest_scrollpos_x, &w->viewport->dest_scrollpos_y);
@@ -4044,29 +4044,40 @@ void UpdateViewportPosition(Window *w)
 		int delta_x = w->viewport->dest_scrollpos_x - w->viewport->scrollpos_x;
 		int delta_y = w->viewport->dest_scrollpos_y - w->viewport->scrollpos_y;
 
+		w->viewport->next_scrollpos_x = w->viewport->scrollpos_x;
+		w->viewport->next_scrollpos_y = w->viewport->scrollpos_y;
+
 		bool update_overlay = false;
 		if (delta_x != 0 || delta_y != 0) {
 			if (_settings_client.gui.smooth_scroll) {
 				int max_scroll = ScaleByMapSize1D(512 * ZOOM_LVL_BASE);
 				/* Not at our desired position yet... */
-				w->viewport->scrollpos_x += Clamp(DivAwayFromZero(delta_x, 4), -max_scroll, max_scroll);
-				w->viewport->scrollpos_y += Clamp(DivAwayFromZero(delta_y, 4), -max_scroll, max_scroll);
+				w->viewport->next_scrollpos_x += Clamp(DivAwayFromZero(delta_x, 4), -max_scroll, max_scroll);
+				w->viewport->next_scrollpos_y += Clamp(DivAwayFromZero(delta_y, 4), -max_scroll, max_scroll);
 			} else {
-				w->viewport->scrollpos_x = w->viewport->dest_scrollpos_x;
-				w->viewport->scrollpos_y = w->viewport->dest_scrollpos_y;
+				w->viewport->next_scrollpos_x = w->viewport->dest_scrollpos_x;
+				w->viewport->next_scrollpos_y = w->viewport->dest_scrollpos_y;
 			}
-			update_overlay = (w->viewport->scrollpos_x == w->viewport->dest_scrollpos_x &&
-								w->viewport->scrollpos_y == w->viewport->dest_scrollpos_y);
+			update_overlay = (w->viewport->next_scrollpos_x == w->viewport->dest_scrollpos_x &&
+								w->viewport->next_scrollpos_y == w->viewport->dest_scrollpos_y);
 		}
+		w->viewport->force_update_overlay_pending = update_overlay;
 
-		ClampViewportToMap(vp, &w->viewport->scrollpos_x, &w->viewport->scrollpos_y);
+		ClampViewportToMap(vp, &w->viewport->next_scrollpos_x, &w->viewport->next_scrollpos_y);
 
 		if (_scrolling_viewport == w) UpdateActiveScrollingViewport(w);
-
-		SetViewportPosition(w, w->viewport->scrollpos_x, w->viewport->scrollpos_y, update_overlay);
 	}
 }
 
+/**
+ * Apply the next viewport position being displayed.
+ * @param w %Window owning the viewport.
+ */
+void ApplyNextViewportPosition(Window *w)
+{
+	SetViewportPosition(w, w->viewport->next_scrollpos_x, w->viewport->next_scrollpos_y, w->viewport->force_update_overlay_pending);
+}
+
 void UpdateViewportSizeZoom(Viewport *vp)
 {
 	vp->dirty_blocks_per_column = CeilDiv(vp->height, vp->GetDirtyBlockHeight());

src/viewport_func.h

@@ -33,7 +33,8 @@ void InitializeWindowViewport(Window *w, int x, int y, int width, int height, ui
 Viewport *IsPtInWindowViewport(const Window *w, int x, int y);
 Point TranslateXYToTileCoord(const Viewport *vp, int x, int y, bool clamp_to_map = true);
 Point GetTileBelowCursor();
-void UpdateViewportPosition(Window *w);
+void UpdateNextViewportPosition(Window *w);
+void ApplyNextViewportPosition(Window *w);
 void UpdateViewportSizeZoom(Viewport *vp);
 
 void MarkViewportDirty(Viewport * const vp, int left, int top, int right, int bottom, ViewportMarkDirtyFlags flags);

src/window.cpp

@@ -3365,11 +3365,16 @@ void UpdateWindows()
 		}
 	}
 
+	for (Window *w : Window::Iterate()) {
+		/* Update viewport only if window is not shaded. */
+		if (w->viewport != nullptr && !w->IsShaded()) UpdateNextViewportPosition(w);
+	}
+
 	DrawDirtyBlocks();
 
 	for (Window *w : Window::Iterate()) {
 		/* Update viewport only if window is not shaded. */
-		if (w->viewport != nullptr && !w->IsShaded()) UpdateViewportPosition(w);
+		if (w->viewport != nullptr && !w->IsShaded()) ApplyNextViewportPosition(w);
 	}
 	ViewportDoDrawProcessAllPending();
 	NetworkDrawChatMessage();

src/window_gui.h

@@ -211,7 +211,7 @@ static const int WHITE_BORDER_DURATION = 3; ///< The initial timeout value for W
  * A viewport is either following a vehicle (its id in then in #follow_vehicle), or it aims to display a specific
  * location #dest_scrollpos_x, #dest_scrollpos_y (#follow_vehicle is then #INVALID_VEHICLE).
  * The actual location being shown is #scrollpos_x, #scrollpos_y.
- * @see InitializeViewport(), UpdateViewportPosition(), UpdateViewportCoordinates().
+ * @see InitializeViewport(), UpdateNextViewportPosition(), ApplyNextViewportPosition(), UpdateViewportCoordinates().
  */
 struct ViewportData : Viewport {
 	VehicleID follow_vehicle; ///< VehicleID to follow if following a vehicle, #INVALID_VEHICLE otherwise.
@@ -219,6 +219,9 @@ struct ViewportData : Viewport {
 	int32 scrollpos_y;        ///< Currently shown y coordinate (virtual screen coordinate of topleft corner of the viewport).
 	int32 dest_scrollpos_x;   ///< Current destination x coordinate to display (virtual screen coordinate of topleft corner of the viewport).
 	int32 dest_scrollpos_y;   ///< Current destination y coordinate to display (virtual screen coordinate of topleft corner of the viewport).
+	int32 next_scrollpos_x;   ///< Next x coordinate to display (virtual screen coordinate of topleft corner of the viewport).
+	int32 next_scrollpos_y;   ///< Next y coordinate to display (virtual screen coordinate of topleft corner of the viewport).
+	bool force_update_overlay_pending; ///< Forced overlay update is pending (see SetViewportPosition)
 };
 
 struct QueryString;