Improve performance of show scrolling viewport on map

src/viewport.cpp

@@ -2784,24 +2784,54 @@ void UpdateViewportPosition(Window *w)
 
 		ClampViewportToMap(vp, w->viewport->scrollpos_x, w->viewport->scrollpos_y);
 
-		if (_scrolling_viewport == w && _settings_client.gui.show_scrolling_viewport_on_map && vp->zoom < ZOOM_LVL_DRAW_MAP) {
+		if (_scrolling_viewport == w) UpdateActiveScrollingViewport(w);
+
+		SetViewportPosition(w, w->viewport->scrollpos_x, w->viewport->scrollpos_y);
+		if (update_overlay) RebuildViewportOverlay(w);
+	}
+}
+
+void UpdateActiveScrollingViewport(Window *w)
+{
+	if (w && (!_settings_client.gui.show_scrolling_viewport_on_map || w->viewport->zoom >= ZOOM_LVL_DRAW_MAP)) w = nullptr;
+
+	const bool bound_valid = (_scrolling_viewport_bound.left != _scrolling_viewport_bound.right);
+
+	if (!w && !bound_valid) return;
+
 	const int gap = ScaleByZoom(1, ZOOM_LVL_MAX);
 
+	auto get_bounds = [&gap](const ViewportData *vp) -> Rect {
 		int lr_low = vp->virtual_left;
-			int lr_hi = w->viewport->scrollpos_x;
+		int lr_hi = vp->dest_scrollpos_x;
 		if (lr_low > lr_hi) Swap(lr_low, lr_hi);
 		int right = lr_hi + vp->virtual_width + gap;
 
 		int tb_low = vp->virtual_top;
-			int tb_hi = w->viewport->scrollpos_y;
+		int tb_hi = vp->scrollpos_y;
 		if (tb_low > tb_hi) Swap(tb_low, tb_hi);
 		int bottom = tb_hi + vp->virtual_height + gap;
 
-			MarkAllViewportMapsDirty(lr_low, tb_low, right, bottom);
-		}
+		return { lr_low, tb_low, right, bottom };
+	};
 
-		SetViewportPosition(w, w->viewport->scrollpos_x, w->viewport->scrollpos_y);
-		if (update_overlay) RebuildViewportOverlay(w);
+	if (w && !bound_valid) {
+		const Rect bounds = get_bounds(w->viewport);
+		MarkAllViewportMapsDirty(bounds.left, bounds.top, bounds.right, bounds.bottom);
+		_scrolling_viewport_bound = bounds;
+	} else if (!w && bound_valid) {
+		const Rect &bounds = _scrolling_viewport_bound;
+		MarkAllViewportMapsDirty(bounds.left, bounds.top, bounds.right, bounds.bottom);
+		_scrolling_viewport_bound = { 0, 0, 0, 0 };
+	} else {
+		/* Calculate symmetric difference of two rectangles */
+		const Rect a = get_bounds(w->viewport);
+		const Rect &b = _scrolling_viewport_bound;
+		if (a.left != b.left) MarkAllViewportMapsDirty(min(a.left, b.left) - gap, min(a.top, b.top) - gap, max(a.left, b.left) + gap, max(a.bottom, b.bottom) + gap);
+		if (a.top != b.top) MarkAllViewportMapsDirty(min(a.left, b.left) - gap, min(a.top, b.top) - gap, max(a.right, b.right) + gap, max(a.top, b.top) + gap);
+		if (a.right != b.right) MarkAllViewportMapsDirty(min(a.right, b.right) - (2 * gap), min(a.top, b.top) - gap, max(a.right, b.right) + gap, max(a.bottom, b.bottom) + gap);
+		if (a.bottom != b.bottom) MarkAllViewportMapsDirty(min(a.left, b.left) - gap, min(a.bottom, b.bottom) - (2 * gap), max(a.right, b.right) + gap, max(a.bottom, b.bottom) + gap);
+		_scrolling_viewport_bound = a;
 	}
 }
 

src/viewport_func.h

@@ -76,6 +76,8 @@ void ViewportDoDraw(const ViewPort *vp, int left, int top, int right, int bottom
 bool ScrollWindowToTile(TileIndex tile, Window *w, bool instant = false);
 bool ScrollWindowTo(int x, int y, int z, Window *w, bool instant = false);
 
+void UpdateActiveScrollingViewport(Window *w);
+
 void RebuildViewportOverlay(Window *w);
 
 bool ScrollMainWindowToTile(TileIndex tile, bool instant = false);

src/window.cpp

@@ -73,6 +73,7 @@ int _scrollbar_size;
 byte _scroller_click_timeout = 0;
 
 Window *_scrolling_viewport; ///< A viewport is being scrolled with the mouse.
+Rect _scrolling_viewport_bound; ///< A viewport is being scrolled with the mouse, the overlay currently covers this viewport rectangle.
 bool _mouse_hovering;      ///< The mouse is hovering over the same point.
 
 SpecialMouseMode _special_mouse_mode; ///< Mode of the mouse.
@@ -1878,6 +1879,7 @@ void InitWindowSystem()
 	_mouseover_last_w = NULL;
 	_last_scroll_window = NULL;
 	_scrolling_viewport = NULL;
+	_scrolling_viewport_bound = { 0, 0, 0, 0 };
 	_mouse_hovering = false;
 
 	NWidgetLeaf::InvalidateDimensionCache(); // Reset cached sizes of several widgets.
@@ -2448,6 +2450,7 @@ static EventState HandleViewportScroll()
 		_cursor.fix_at = false;
 		_scrolling_viewport = NULL;
 		_last_scroll_window = NULL;
+		UpdateActiveScrollingViewport(nullptr);
 		return ES_NOT_HANDLED;
 	}
 

src/window_gui.h

@@ -892,6 +892,7 @@ extern int _scrollbar_size;
 extern byte _scroller_click_timeout;
 
 extern Window *_scrolling_viewport;
+extern Rect _scrolling_viewport_bound;
 extern bool _mouse_hovering;
 
 /** Mouse modes. */