diff --git a/ui/dr-ui/src/develop.rs b/ui/dr-ui/src/develop.rs
index b8109f9..c574993 100644
--- a/ui/dr-ui/src/develop.rs
+++ b/ui/dr-ui/src/develop.rs
@@ -2449,35 +2449,28 @@ impl DevelopSession {
             .map(|f| (f.stock.as_str(), f.print.is_some()))
     }
 
-    /// Re-bake if `op` is the film, and do nothing otherwise.
+    /// Re-bake if `op_index` names the film, and do nothing otherwise.
     ///
-    /// The test is here rather than at the call site so that the callback in
+    /// `op_index` counts over [`Self::scoped_capabilities`] — the same list
+    /// [`Self::lookup`] resolves a slider through — so that is the only list to
+    /// ask. An earlier version also indexed `rows()`, which is one entry per
+    /// *parameter* and filtered by the active tab: past its end the check
+    /// short-circuited, the tables were never rebuilt, and the film's own
+    /// sliders moved nothing at all.
+    ///
+    /// The test is here rather than at the call site so the callback in
     /// `lib.rs` goes on naming no operation, which is the rule the whole panel
     /// is built on (ARCH §4.3a).
-    pub fn rebake_film_if_affected(&mut self, op: i32) {
-        let is_film = self
-            .rows()
-            .get(op.max(0) as usize)
-            .map(|_| ())
-            .and(
-                self.scoped_capabilities()
-                    .get(op.max(0) as usize)
-                    .map(|c| c.id == dr_pipeline::ops::film_sim::ID),
-            )
-            .unwrap_or(false);
+    pub fn rebake_film_if_affected(&mut self, op_index: i32) {
+        let is_film = usize::try_from(op_index)
+            .ok()
+            .and_then(|i| self.scoped_capabilities().get(i).map(|c| c.id))
+            .is_some_and(|id| id == dr_pipeline::ops::film_sim::ID);
         if is_film {
             self.rebake_film();
         }
     }
 
-    /// Re-bake the current stock.
-    ///
-    /// Called when the film's own exposure sliders move: those ride *inside*
-    /// the baked tables rather than arriving as uniforms, because the print
-    /// balance is solved against them — an enlarger's filtration depends on
-    /// how the negative was exposed. So a slider drag has to rebuild the
-    /// lookup, which is why the bake is measured in milliseconds and not in
-    /// frames.
     /// The stock, the paper, and how far it was developed.
     ///
     /// Push rides with the other two through every path that re-bakes, because