Add Alt+click on timeline to hide all layers except the clicked one

https://community.aseprite.org/t/turn-off-all-other-layers/151

src/app/file/ase_format.cpp

@@ -551,7 +551,8 @@ static void ase_file_write_layer_chunk(FILE* f, dio::AsepriteFrameHeader* frame_
   ChunkWriter chunk(f, frame_header, ASE_FILE_CHUNK_LAYER);
 
   // Flags
-  fputw(static_cast<int>(layer->flags()), f);
+  fputw(static_cast<int>(layer->flags()) &
+        static_cast<int>(doc::LayerFlags::PersistentFlagsMask), f);
 
   // Layer type
   fputw((layer->isImage() ? ASE_FILE_LAYER_IMAGE:

src/app/ui/timeline/timeline.cpp

@@ -769,8 +769,48 @@ bool Timeline::onProcessMessage(Message* msg)
           if (validLayer(m_clk.layer)) {
             Row& row = m_rows[m_clk.layer];
             Layer* layer = row.layer();
-            ASSERT(layer);
-            if (layer->isVisible())
+            ASSERT(layer)
+
+            // Hide everything or restore alternative state
+            bool oneWithInternalState = false;
+            if (msg->altPressed()) {
+              for (const Row& row : m_rows) {
+                const Layer* l = row.layer();
+                if (l->hasFlags(LayerFlags::Internal_WasVisible)) {
+                  oneWithInternalState = true;
+                  break;
+                }
+              }
+
+              // If there is one layer with the internal state, restore the previous visible state
+              if (oneWithInternalState) {
+                for (Row& row : m_rows) {
+                  Layer* l = row.layer();
+                  if (l->hasFlags(LayerFlags::Internal_WasVisible)) {
+                    l->setVisible(true);
+                    l->switchFlags(LayerFlags::Internal_WasVisible, false);
+                  }
+                  else {
+                    l->setVisible(false);
+                  }
+                }
+              }
+              // In other case, hide everything
+              else {
+                for (Row& row : m_rows) {
+                  Layer* l = row.layer();
+                  l->switchFlags(LayerFlags::Internal_WasVisible, l->isVisible());
+                  l->setVisible(false);
+                }
+              }
+
+              regenerateRows();
+              invalidate();
+
+              m_document->notifyGeneralUpdate();
+            }
+
+            if (layer->isVisible() && !oneWithInternalState)
               m_state = STATE_HIDING_LAYERS;
             else
               m_state = STATE_SHOWING_LAYERS;

src/dio/aseprite_decoder.cpp

@@ -417,7 +417,9 @@ doc::Layer* AsepriteDecoder::readLayerChunk(AsepriteHeader* header,
 
   if (layer) {
     // flags
-    layer->setFlags(static_cast<doc::LayerFlags>(flags));
+    layer->setFlags(static_cast<doc::LayerFlags>(
+                      flags &
+                      static_cast<int>(doc::LayerFlags::PersistentFlagsMask)));
 
     // name
     layer->setName(name.c_str());

src/doc/layer.h

@@ -39,6 +39,10 @@ namespace doc {
     Collapsed  = 32,            // Prefer to show this group layer collapsed
     Reference  = 64,            // Is a reference layer
 
+    PersistentFlagsMask = 0xffff,
+
+    Internal_WasVisible = 0x10000, // Was visible in the alternative state (Alt+click)
+
     BackgroundLayerFlags = LockMove | Background,
   };