diff --git a/crates/engine/src/media.rs b/crates/engine/src/media.rs
index 6a28a5fe3..67367e0ea 100644
--- a/crates/engine/src/media.rs
+++ b/crates/engine/src/media.rs
@@ -403,6 +403,15 @@ impl MediaCache {
         (self.thumbs.len(), self.thumbs.cost())
     }
 
+    /// Whether the decoded source of `id` at `level` is held.
+    pub fn has_source(&self, id: PhotoId, level: SourceLevel) -> bool {
+        match level {
+            SourceLevel::Thumb => self.thumbs.contains(&id),
+            SourceLevel::Preview => self.previews.iter().any(|e| e.0 == id),
+            SourceLevel::Full => self.full.as_ref().is_some_and(|e| e.0 == id),
+        }
+    }
+
     /// Decoded sources held: (thumbnail level, preview level, full size).
     pub fn usage(&self) -> (crate::memory::Usage, crate::memory::Usage, crate::memory::Usage) {
         use crate::memory::Usage;
@@ -1389,6 +1398,20 @@ mod tests {
         assert!(mean(&a) > mean(&b) + 10.0, "two stops brighter after: {} vs {}", mean(&a), mean(&b));
     }
 
+    #[test]
+    fn has_source_follows_what_the_session_accepted() {
+        let mut s = crate::Session::with_demo();
+        let id = s.active().unwrap();
+        let p = s.catalog.photo(id).unwrap().clone();
+        let long = p.width.max(p.height) as usize;
+        assert!(!s.media.has_source(id, SourceLevel::Full));
+        let r = s.render_job(id, long, long, false, true).unwrap().run();
+        s.accept(&r);
+        assert!(s.media.has_source(id, SourceLevel::Full));
+        assert!(!s.media.has_source(id, SourceLevel::Preview));
+        assert!(!s.media.has_source(PhotoId(id.0 + 1), SourceLevel::Full));
+    }
+
     #[test]
     fn variant_jobs_have_their_own_keys_and_hit_the_cache() {
         let mut s = crate::Session::with_demo();
diff --git a/crates/ui-egui/src/panels/detail.rs b/crates/ui-egui/src/panels/detail.rs
index 32bf919b6..c9fb643d4 100644
--- a/crates/ui-egui/src/panels/detail.rs
+++ b/crates/ui-egui/src/panels/detail.rs
@@ -266,6 +266,11 @@ fn request_window(app: &mut LightcraftApp, c: &WindowCtx, v: &WindowView) -> Opt
         to_px * (v.visible.bottom() - v.target.top()),
     );
     let win = crate::region::window_for(fw, fh, visible, crate::region::max_span(c.texture_side))?;
+    // the Before side waits for the original the After is decoding (see `defer_before_window`)
+    let original_held = app.session.media.has_source(c.id, lightcraft_engine::SourceLevel::for_size(fw.max(fh)));
+    if crate::region::defer_before_window(v.before, app.renderer.is_pending(Slot::Region), original_held) {
+        return app.region_before_view.filter(|w| w.photo == c.id);
+    }
     let job = if v.before {
         app.session.region_job_before(c.id, fw, fh, win, !c.crop_tool)
     } else {
diff --git a/crates/ui-egui/src/region.rs b/crates/ui-egui/src/region.rs
index 031db8ebe..db170da4b 100644
--- a/crates/ui-egui/src/region.rs
+++ b/crates/ui-egui/src/region.rs
@@ -170,6 +170,13 @@ pub fn window_clip(mode: crate::state::BeforeAfter, before_side: bool, pane: egu
     clip.intersect(canvas)
 }
 
+/// Whether the Before window waits for the After's. Both are cut from the same decoded original;
+/// asked for together while it isn't held yet, each worker decodes its own copy (two full-size
+/// decodes, two uploads, one thrown away). The Before follows once the original is in.
+pub fn defer_before_window(before_side: bool, after_pending: bool, original_held: bool) -> bool {
+    before_side && after_pending && !original_held
+}
+
 /// Whether the picture on screen has the frame's aspect (to the one pixel a texture's whole-pixel
 /// size is off by): a window is a part of the frame, so it can only be placed over a picture that
 /// is the frame (an unsupported raw's embedded JPEG may be cropped differently).
@@ -403,6 +410,16 @@ mod tests {
         assert_eq!(window_clip(Original, true, canvas, canvas, img), canvas);
     }
 
+    // Given Before and After windows asked for at once at first open, only the After's goes: the
+    // Before's waits for the decoded original instead of decoding another copy
+    #[test]
+    fn the_before_window_waits_for_the_original_the_after_is_decoding() {
+        assert!(defer_before_window(true, true, false));
+        assert!(!defer_before_window(true, true, true), "the original is in");
+        assert!(!defer_before_window(true, false, false), "nothing is decoding it");
+        assert!(!defer_before_window(false, true, false), "the After never waits");
+    }
+
     // Hostile numbers give a plan, never a panic
     #[test]
     fn hostile_sizes_give_a_usable_plan() {