diff --git a/crates/ui-egui/src/app.rs b/crates/ui-egui/src/app.rs
index bf5964ba6..518f4e809 100644
--- a/crates/ui-egui/src/app.rs
+++ b/crates/ui-egui/src/app.rs
@@ -99,6 +99,7 @@ impl EffectcraftApp {
             footage_watch_key: None,
             lut_watcher: None,
             lut_generation: effectcraft_engine::lut_reload::generation(),
+            purges: 0,
             device_lost_close: None,
             dialog_answers: std::sync::mpsc::channel(),
             file_dialogs: FileDialogsUp::default(),
diff --git a/crates/ui-egui/src/app/frame.rs b/crates/ui-egui/src/app/frame.rs
index 488b3c5ef..b78c38c73 100644
--- a/crates/ui-egui/src/app/frame.rs
+++ b/crates/ui-egui/src/app/frame.rs
@@ -151,7 +151,11 @@ impl EffectcraftApp {
                         self.ui.status = e;
                     }
                 }
-                effectcraft_engine::Event::PurgeCaches => self.frames.clear(),
+                effectcraft_engine::Event::PurgeCaches => {
+                    self.frames.clear();
+                    // (frames made after it get keys of their own: see `Self::purge_stamp`)
+                    self.purges = self.purges.wrapping_add(1);
+                }
                 effectcraft_engine::Event::CopyImage { width, height, rgba } => {
                     if let Err(e) = copy_image(ctx, width, height, &rgba) {
                         self.ui.status = e;
diff --git a/crates/ui-egui/src/app/viewer_frames.rs b/crates/ui-egui/src/app/viewer_frames.rs
index a76c88b2d..d9e6e84f2 100644
--- a/crates/ui-egui/src/app/viewer_frames.rs
+++ b/crates/ui-egui/src/app/viewer_frames.rs
@@ -128,7 +128,7 @@ impl EffectcraftApp {
             frame: 0,
             scale: (scale * 1000.0).round() as u32,
             view: self.view_hash(comp),
-            opts: frames::opts_hash(&self.frame_opts(comp, scale)) ^ self.roto_model_stamp() ^ self.lut_stamp(),
+            opts: frames::opts_hash(&self.frame_opts(comp, scale)) ^ self.roto_model_stamp() ^ self.lut_stamp() ^ self.purge_stamp(),
         }
     }
 
@@ -149,6 +149,14 @@ impl EffectcraftApp {
         self.lut_generation.wrapping_mul(0x9e37_79b9_7f4a_7c15)
     }
 
+    /// Changes when the caches are purged (Event::PurgeCaches: footage reloaded, Purge): the
+    /// frames on screen and the ones still rendering may show the files as they were, and a reload
+    /// that changes no metadata keeps the project's revision, so the new frames must not share
+    /// their keys (#3539; the viewers show a frame again only when its key changes). 0 until then.
+    fn purge_stamp(&self) -> u64 {
+        self.purges.wrapping_mul(0xc2b2_ae3d_27d4_eb4f)
+    }
+
     /// The frame series the viewer of `comp` shows now (its scale, view and options, for the
     /// comp's current content): what the cache bars count.
     pub fn shown_series(&self, comp: ItemId) -> FrameKey {
diff --git a/crates/ui-egui/src/lib.rs b/crates/ui-egui/src/lib.rs
index 699f68e0f..1558ce743 100644
--- a/crates/ui-egui/src/lib.rs
+++ b/crates/ui-egui/src/lib.rs
@@ -377,6 +377,8 @@ pub struct EffectcraftApp {
     lut_watcher: Option<effectcraft_engine::lut_reload::Watcher>,
     /// The LUT generation the frames kept were rendered at ([`effectcraft_engine::lut_reload::generation`]).
     lut_generation: u64,
+    /// How many times the caches were purged ([`Self::purge_stamp`]).
+    purges: u64,
     /// The graphics device was lost and the window is closing (see `close_after_device_loss`);
     /// `true` keeps crash recovery for the next launch.
     device_lost_close: Option<bool>,