diff --git a/src/gfx.cpp b/src/gfx.cpp
index 30458d6171..9145fa449d 100644
--- a/src/gfx.cpp
+++ b/src/gfx.cpp
@@ -1753,7 +1753,6 @@ void DrawDirtyBlocks()
 				DrawOverlappedWindowFlags flags = DOWF_MARK_DIRTY;
 				if (unlikely(HasBit(_gfx_debug_flags, GDF_SHOW_WINDOW_DIRTY))) {
 					flags |= DOWF_SHOW_DEBUG;
-					_dirty_block_colour.fetch_add(1, std::memory_order_relaxed);
 				}
 				DrawOverlappedWindowWithClipping(w, w->left, w->top, w->left + w->width, w->top + w->height, flags);
 				w->flags &= ~(WF_DIRTY | WF_WIDGETS_DIRTY);
@@ -1765,7 +1764,6 @@ void DrawDirtyBlocks()
 						DrawOverlappedWindowFlags flags = DOWF_MARK_DIRTY;
 						if (unlikely(HasBit(_gfx_debug_flags, GDF_SHOW_WIDGET_DIRTY))) {
 							flags |= DOWF_SHOW_DEBUG;
-							_dirty_block_colour.fetch_add(1, std::memory_order_relaxed);
 						}
 						DrawOverlappedWindowWithClipping(w, w->left + widget->pos_x, w->top + widget->pos_y, w->left + widget->pos_x + widget->current_x, w->top + widget->pos_y + widget->current_y, flags);
 					}
diff --git a/src/viewport.cpp b/src/viewport.cpp
index 0fed27191d..7f8b420b44 100644
--- a/src/viewport.cpp
+++ b/src/viewport.cpp
@@ -2215,7 +2215,7 @@ void ViewportDrawDirtyBlocks(const DrawPixelInfo *dpi, bool increment_colour)
 	int bottom = UnScaleByZoom(dpi->height, dpi->zoom);
 
 	const uint dirty_block_colour = increment_colour ? _dirty_block_colour.fetch_add(1, std::memory_order_relaxed) : _dirty_block_colour.load(std::memory_order_relaxed);
-	int colour = _string_colourmap[dirty_block_colour];
+	int colour = _string_colourmap[dirty_block_colour & 0xF];
 
 	dst = dpi->dst_ptr;
 
diff --git a/src/window.cpp b/src/window.cpp
index d5ea47a7c9..a4cd191795 100644
--- a/src/window.cpp
+++ b/src/window.cpp
@@ -948,8 +948,9 @@ void DrawOverlappedWindow(Window *w, int left, int top, int right, int bottom, D
 	dp->zoom = ZOOM_LVL_NORMAL;
 	w->OnPaint();
 	if (unlikely(flags & DOWF_SHOW_DEBUG)) {
+		if (w->viewport != nullptr) ViewportDoDrawProcessAllPending();
 		extern void ViewportDrawDirtyBlocks(const DrawPixelInfo *dpi, bool increment_colour);
-		ViewportDrawDirtyBlocks(_cur_dpi, false);
+		ViewportDrawDirtyBlocks(_cur_dpi, true);
 	}
 	if (flags & DOWF_MARK_DIRTY) {
 		VideoDriver::GetInstance()->MakeDirty(left, top, right - left, bottom - top);