Fix regression breaking linked cels on "Merge Down" (fix #4685)

src/app/cmd/flatten_layers.cpp

@@ -1,5 +1,5 @@
 // Aseprite
-// Copyright (C) 2019-2022  Igara Studio S.A.
+// Copyright (C) 2019-2024  Igara Studio S.A.
 // Copyright (C) 2001-2018  David Capello
 //
 // This program is distributed under the terms of
@@ -35,6 +35,8 @@
 #include "doc/sprite.h"
 #include "render/render.h"
 
+#include <algorithm>
+
 namespace app {
 namespace cmd {
 
@@ -128,11 +130,24 @@ void FlattenLayers::onExecute()
     RestoreVisibleLayers restore;
     restore.showSelectedLayers(sprite, layers);
 
+    const LayerList visibleLayers = sprite->allVisibleLayers();
+
     // Map draw area to image coords
     const gfx::ClipF area_to_image(0, 0, area);
 
     // Copy all frames to the background.
     for (frame_t frame(0); frame<sprite->totalFrames(); ++frame) {
+      // If the flatLayer is the only cel in this frame, we can skip
+      // this frame to keep existing links in the flatLayer.
+      const bool anotherCelExists =
+        std::any_of(visibleLayers.begin(),
+                    visibleLayers.end(),
+                    [flatLayer, frame](const Layer* other) {
+                      return (flatLayer != other && other->cel(frame));
+                    });
+      if (!anotherCelExists)
+        continue;
+
       // Clear the image and render this frame.
       clear_image(image.get(), bgcolor);
       render.renderSprite(image.get(), sprite, frame, area_to_image);

tests/scripts/merge_down_bugs.lua

@@ -1,4 +1,4 @@
--- Copyright (C) 2019  Igara Studio S.A.
+-- Copyright (C) 2019-2024  Igara Studio S.A.
 --
 -- This file is released under the terms of the MIT license.
 -- Read LICENSE.txt for more information.
@@ -46,3 +46,25 @@ do
   local after = s.cels[1].image
   assert(before:isEqual(after))
 end
+
+-- Check that linked cels are not broken (regression in issue #4685)
+-- We create two layers, the bottom one with 4 linked frames, and the
+-- top one with one cel at 2nd frame, when we merge them, the
+-- resulting layer should have frame 1, 3, and 4 linked.
+do
+  local s = Sprite(32, 32)
+  app.useTool{ color=Color(255, 0, 0), points={ {0,0}, {32,32} } }
+  app.layer.isContinuous = true
+  app.command.NewFrame{ content=cellinked }
+  app.command.NewFrame{ content=cellinked }
+  app.command.NewFrame{ content=cellinked }
+  s:newLayer()
+  app.frame = 2
+  app.useTool{ color=Color(0, 0, 255), points={ {32,0}, {0,32} } }
+  app.command.MergeDownLayer()
+  local cels = app.layer.cels
+  -- Check that frame 1, 3, and 4 have the same image (linked cels)
+  assert(cels[1].image ~= cels[2].image)
+  assert(cels[1].image == cels[3].image)
+  assert(cels[1].image == cels[4].image)
+end