diff --git a/ui/dr-ui/src/merge_thumbs.rs b/ui/dr-ui/src/merge_thumbs.rs
index 9d05d31..6c9d296 100644
--- a/ui/dr-ui/src/merge_thumbs.rs
+++ b/ui/dr-ui/src/merge_thumbs.rs
@@ -228,7 +228,7 @@ mod tests {
         // (0,0),(1,0),(0,1),(1,1) → 0, 10, 1, 11: mean 5.5, rounded.
         assert_eq!(at(0, 0), 6);
         // The right-hand column is one pixel wide: (4,0) and (4,1).
-        assert_eq!(at(2, 0), (40 + 41 + 1) / 2);
+        assert_eq!(at(2, 0), 41, "40 and 41, rounded");
         // The bottom-right is a single pixel.
         assert_eq!(at(2, 1), 42);
         assert!(out.chunks_exact(3).all(|p| p[2] == 1000));
diff --git a/ui/dr-ui/src/merge_ui.rs b/ui/dr-ui/src/merge_ui.rs
index ceba979..8d74c25 100644
--- a/ui/dr-ui/src/merge_ui.rs
+++ b/ui/dr-ui/src/merge_ui.rs
@@ -97,13 +97,16 @@ impl MergeController {
     }
 }
 
+/// The names a library folder holds, by folder.
+pub type NamesIn = Box<dyn Fn(&str) -> std::collections::HashSet<String>>;
+
 /// What the page needs from the library to start: the account for the
 /// fetch, where the outbox is, and the names a library folder already
 /// holds, so the composite is not named over one of them.
 pub struct Context {
     pub conn: dr_sync::Connection,
     pub outbox: std::path::PathBuf,
-    pub names_in: Box<dyn Fn(&str) -> std::collections::HashSet<String>>,
+    pub names_in: NamesIn,
 }
 
 /// Wire the page. `sources` yields the selection as fetchable library