M4.8: Edit ▸ Purge clears what it names: disk and snapshot purges keep the RAM preview, memory purges drop decoded footage

- Every purge emptied the viewers' RAM preview; Purge ▸ All Disk Cache and Snapshot now leave
  it (Image Cache Memory, All Memory, All Memory & Disk Cache, All and All 3D Cache still
  clear it).
- The memory purges also drop the decoded footage frames (`FootageSource::purge`, the media
  pool's `clear_frames`; decoders stay open), so footage reads decode again.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/commands/edit.rs

@@ -569,6 +569,7 @@ fn purge_caches(s: &mut Session, p: &Value) -> Result<Value> {
     let disk = matches!(what.as_str(), "all" | "memoryAndDisk" | "disk");
     if memory {
         s.layer_cache.clear();
+        s.footage.purge();
     }
     let mut disk_entries = 0;
     if disk && let Some(dc) = &s.disk_cache {
@@ -581,7 +582,11 @@ fn purge_caches(s: &mut Session, p: &Value) -> Result<Value> {
     if matches!(what.as_str(), "all" | "image" | "memory" | "memoryAndDisk") {
         effectcraft_effects::roto::purge();
     }
-    s.events.push(crate::Event::PurgeCaches);
+    // The viewers' RAM preview goes with the memory purges (and the 3D one, whose renders it
+    // holds); purging the disk cache or the snapshot leaves it.
+    if memory || what == "3d" {
+        s.events.push(crate::Event::PurgeCaches);
+    }
     s.toast(format!("Purged {what} cache"));
     Ok(json!({"purged": what, "diskEntries": disk_entries}))
 }

crates/engine/src/tests_menu_cmds.rs

@@ -263,9 +263,35 @@ fn label_group_purge_and_edit_original() {
     s.execute("layer.select", json!({"layers": [a]})).unwrap();
     s.execute("edit.selectLabelGroup", json!({})).unwrap();
     assert_eq!(s.state.selected_layers.len(), 2);
+    // A footage source that counts purges (the media pool drops its decoded frames).
+    struct Counting(std::sync::atomic::AtomicUsize);
+    impl crate::render::FootageSource for Counting {
+        fn frame(
+            &self,
+            _: effectcraft_project::ItemId,
+            _: &effectcraft_project::Footage,
+            _: effectcraft_time::Tick,
+        ) -> Option<std::sync::Arc<crate::render::Image>> {
+            None
+        }
+        fn purge(&self) {
+            self.0.fetch_add(1, std::sync::atomic::Ordering::SeqCst);
+        }
+    }
+    let src = std::sync::Arc::new(Counting(Default::default()));
+    s.footage = src.clone();
     s.drain_events();
     s.execute("edit.purge", json!({"what": "memory"})).unwrap();
     assert!(s.drain_events().contains(&Event::PurgeCaches));
+    assert_eq!(src.0.load(std::sync::atomic::Ordering::SeqCst), 1, "decoded footage frames are dropped");
+    // The disk cache and the snapshot purge alone: the RAM preview stays.
+    for what in ["disk", "snapshot"] {
+        s.execute("edit.purge", json!({"what": what})).unwrap();
+        assert!(!s.drain_events().contains(&Event::PurgeCaches), "{what}");
+    }
+    assert_eq!(src.0.load(std::sync::atomic::Ordering::SeqCst), 1);
+    s.execute("edit.purge", json!({"what": "3d"})).unwrap();
+    assert!(s.drain_events().contains(&Event::PurgeCaches));
     assert!(!s.is_enabled("edit.editOriginal"));
 }
 

crates/media/src/pool.rs

@@ -233,12 +233,16 @@ impl MediaPool {
         }
     }
 
-    /// Drop every cached frame and decoder.
-    pub fn clear(&self) {
+    /// Drop every cached frame (decoders stay open).
+    pub fn clear_frames(&self) {
         let mut c = lock(&self.inner.cache);
         c.lru = Lru::default();
         c.last.clear();
-        drop(c);
+    }
+
+    /// Drop every cached frame and decoder.
+    pub fn clear(&self) {
+        self.clear_frames();
         lock(&self.inner.sources).clear();
     }
 
@@ -602,6 +606,9 @@ impl FootageSource for MediaPool {
     fn set_cache_budget(&self, bytes: usize) {
         self.set_budget(bytes);
     }
+    fn purge(&self) {
+        self.clear_frames();
+    }
     fn cache_budget(&self) -> Option<usize> {
         Some(self.budget())
     }

crates/render/src/lib.rs

@@ -56,6 +56,9 @@ pub trait FootageSource: Send + Sync {
     /// Set the decoded-frame cache budget in bytes (Settings ▸ Memory & CPU); sources without
     /// a cache ignore it.
     fn set_cache_budget(&self, _bytes: usize) {}
+    /// Drop the decoded frames it keeps (Edit ▸ Purge ▸ All Memory / Image Cache Memory): the
+    /// next reads decode again. Sources without a cache ignore it.
+    fn purge(&self) {}
     /// Settings ▸ Disk ▸ Conformed Audio Folder: where decoded audio is kept between reads
     /// (`None` = off); sources that don't decode audio ignore it.
     fn set_conform_folder(&self, _folder: Option<std::path::PathBuf>) {}