diff --git a/library/alloc/src/collections/btree/append.rs b/library/alloc/src/collections/btree/append.rs
index 9c53c84f5b2fb..1d6488dd2dfe3 100644
--- a/library/alloc/src/collections/btree/append.rs
+++ b/library/alloc/src/collections/btree/append.rs
@@ -81,15 +81,18 @@ impl<K, V> Root<K, V> {
             // the appended elements even if advancing the iterator panicks.
             *length += 1;
         }
-        self.fix_right_edge();
+        self.fix_right_border_of_plentiful();
     }
 
-    fn fix_right_edge(&mut self) {
-        // Handle underfull nodes, start from the top.
+    /// Stock up any underfull nodes on the right border of the tree.
+    /// The other nodes, those that are not the root nor a rightmost edge,
+    /// must have MIN_LEN elements to spare.
+    fn fix_right_border_of_plentiful(&mut self) {
         let mut cur_node = self.borrow_mut();
         while let Internal(internal) = cur_node.force() {
             // Check if right-most child is underfull.
             let mut last_kv = internal.last_kv().consider_for_balancing();
+            debug_assert!(last_kv.left_child_len() >= MIN_LEN * 2);
             let right_child_len = last_kv.right_child_len();
             if right_child_len < MIN_LEN {
                 // We need to steal.
diff --git a/library/alloc/src/collections/btree/map/tests.rs b/library/alloc/src/collections/btree/map/tests.rs
index 26f8f9da7ec82..f92aed8ce15bf 100644
--- a/library/alloc/src/collections/btree/map/tests.rs
+++ b/library/alloc/src/collections/btree/map/tests.rs
@@ -1821,7 +1821,6 @@ fn test_append_ord_chaos() {
 }
 
 fn rand_data(len: usize) -> Vec<(u32, u32)> {
-    assert!(len * 2 <= 70029); // from that point on numbers repeat
     let mut rng = DeterministicRng::new();
     Vec::from_iter((0..len).map(|_| (rng.next(), rng.next())))
 }
@@ -1886,6 +1885,25 @@ fn test_split_off_tiny_right_height_2() {
     assert_eq!(*right.last_key_value().unwrap().0, last);
 }
 
+#[test]
+fn test_split_off_halfway() {
+    let mut rng = DeterministicRng::new();
+    for &len in &[NODE_CAPACITY, 25, 50, 75, 100] {
+        let mut data = Vec::from_iter((0..len).map(|_| (rng.next(), ())));
+        // Insertion in non-ascending order creates some variation in node length.
+        let mut map = BTreeMap::from_iter(data.iter().copied());
+        data.sort();
+        let small_keys = data.iter().take(len / 2).map(|kv| kv.0);
+        let large_keys = data.iter().skip(len / 2).map(|kv| kv.0);
+        let split_key = large_keys.clone().next().unwrap();
+        let right = map.split_off(&split_key);
+        map.check();
+        right.check();
+        assert!(map.keys().copied().eq(small_keys));
+        assert!(right.keys().copied().eq(large_keys));
+    }
+}
+
 #[test]
 fn test_split_off_large_random_sorted() {
     // Miri is too slow
diff --git a/library/alloc/src/collections/btree/mod.rs b/library/alloc/src/collections/btree/mod.rs
index ebcbb0e467c46..cdb39104047f0 100644
--- a/library/alloc/src/collections/btree/mod.rs
+++ b/library/alloc/src/collections/btree/mod.rs
@@ -38,6 +38,7 @@ pub unsafe fn unwrap_unchecked<T>(val: Option<T>) -> T {
 #[cfg(test)]
 /// XorShiftRng
 struct DeterministicRng {
+    count: usize,
     x: u32,
     y: u32,
     z: u32,
@@ -47,11 +48,13 @@ struct DeterministicRng {
 #[cfg(test)]
 impl DeterministicRng {
     fn new() -> Self {
-        DeterministicRng { x: 0x193a6754, y: 0xa8a7d469, z: 0x97830e05, w: 0x113ba7bb }
+        DeterministicRng { count: 0, x: 0x193a6754, y: 0xa8a7d469, z: 0x97830e05, w: 0x113ba7bb }
     }
 
-    /// Guarantees that the first 70029 results are unique.
+    /// Guarantees that each returned number is unique.
     fn next(&mut self) -> u32 {
+        self.count += 1;
+        assert!(self.count <= 70029);
         let x = self.x;
         let t = x ^ (x << 11);
         self.x = self.y;
diff --git a/library/alloc/src/collections/btree/set/tests.rs b/library/alloc/src/collections/btree/set/tests.rs
index 4d05bc4ebfa1e..fd19c0078a748 100644
--- a/library/alloc/src/collections/btree/set/tests.rs
+++ b/library/alloc/src/collections/btree/set/tests.rs
@@ -696,8 +696,10 @@ fn test_first_last() {
     assert_eq!(a.pop_last(), None);
 }
 
+// Unlike the function with the same name in map/tests, returns no values.
+// Which also means it returns different predetermined pseudo-random keys,
+// and the test cases using this function explore slightly different trees.
 fn rand_data(len: usize) -> Vec<u32> {
-    assert!(len <= 70029); // from that point on numbers repeat
     let mut rng = DeterministicRng::new();
     Vec::from_iter((0..len).map(|_| rng.next()))
 }
diff --git a/library/alloc/src/collections/btree/split.rs b/library/alloc/src/collections/btree/split.rs
index 6108c139bb3a6..4561c8eaf47fb 100644
--- a/library/alloc/src/collections/btree/split.rs
+++ b/library/alloc/src/collections/btree/split.rs
@@ -53,6 +53,9 @@ impl<K, V> Root<K, V> {
         }
     }
 
+    /// Stock up or merge away any underfull nodes on the right border of the
+    /// tree. The other nodes, those that are not the root nor a rightmost edge,
+    /// must already have at least MIN_LEN elements.
     fn fix_right_border(&mut self) {
         self.fix_top();
 
@@ -72,6 +75,7 @@ impl<K, V> Root<K, V> {
                     }
                     cur_node = last_kv.into_right_child();
                 }
+                debug_assert!(cur_node.len() > MIN_LEN);
             }
         }
 
@@ -98,6 +102,7 @@ impl<K, V> Root<K, V> {
                     }
                     cur_node = first_kv.into_left_child();
                 }
+                debug_assert!(cur_node.len() > MIN_LEN);
             }
         }