Detect broken child mark-refs; restore the timeline/top divider

- scene: point-frame rejects out-of-range offsets so broken-marks catches
  boundary shifts and cascades, not just deleted refs; add broken-reason
- subs: annotations carry :broken/:reason and sort broken ones to the bottom
- views: ⚠ on broken annotations (hover = reason); drag-resize the top/timeline
  split via --top-h (vh)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tl/src/tl/scene.cljs

@@ -97,8 +97,9 @@
 
     (map? point)
     (when-let [{:keys [xs xe track]} (target-range scene (:ref point))]
-      (let [n (:at point)]
+      (let [n (:at point)
-        {:frame (if (neg? n) (+ xe n 1) (+ xs n)) :track track}))))
+            f (if (neg? n) (+ xe n 1) (+ xs n))]
+        (when (<= xs f xe) {:frame f :track track})))))   ; nil if trimmed out of range
 
 (defn resolve-mark
   "One mark → its segment(s) (1 for a ref mark, 1+ for an absolute mark), with
@@ -141,6 +142,13 @@
        (filterv (fn [m] (empty? (resolve-mark scene gid m))))
        (mapv :id)))
 
+(defn broken-reason
+  "A short why for `gid`'s first broken mark, or nil if nothing's broken."
+  [scene gid]
+  (when-let [mid (first (broken-marks scene gid))]
+    (let [ref (->> (:marks (grp scene gid)) (some #(when (= mid (:id %)) %)) :start :ref)]
+      (if (target-range scene ref) "reference trimmed away" "referenced clip deleted"))))
+
 ;; --- what the current timeline is made of --------------------------------
 
 (defn content-segments

tl/src/tl/subs.cljs

@@ -62,13 +62,15 @@
           (keep (fn [[gid g]]
                   (when (and (= :annotation (:type g)) (= ctx (:parent g)))
                     (let [src-segs (scene/resolve scene gid)
-                          bars     (vec (mapcat (fn [{[a b] :src}] (scene/pieces segs a b)) src-segs))]
+                          bars     (vec (mapcat (fn [{[a b] :src}] (scene/pieces segs a b)) src-segs))
+                          reason   (scene/broken-reason scene gid)]
                       {:id gid :name (:name g) :color (or (:color g) "#4e8fc2")
                        :content (:content g) :children (count (:marks g))
                        :nested (get nested gid 0)
                        :draft (boolean (:draft g))
+                       :broken (boolean reason) :reason reason
                        :start (or (ffirst bars) 0) :bars bars}))))
-          (sort-by :start)
+          (sort-by (juxt :broken :start))      ; broken annotations sink to the bottom
           vec))))
 
 ;; the annotation the playhead is currently inside (or the latest one passed) —

tl/src/tl/views.cljs

@@ -228,7 +228,7 @@
               ^{:key (:id a)}
               [:div.ann {:id (str "ann-" (name (:id a))) :class (when (= (:id a) active) "active")}
                [:div.ann-head
-                [:div.ann-title (:name a)]
+                [:div.ann-title (when (:broken a) [:span {:title (:reason a)} "⚠ "]) (:name a)]
                 [:div.ann-actions
                  [jump-control open a]
                  [:button.expand-btn {:title "Expand" :on-click #(rf/dispatch [::events/expand (:id a)])}
@@ -324,6 +324,17 @@
       [:input {:type "range" :min 8 :max 120 :value row-h
                :on-change #(rf/dispatch [::events/set-row-h (js/parseFloat (.. % -target -value))])}]]]))
 
+(defn- drag-top!
+  "Drag the horizontal divider: set --top-h (in vh) from the pointer's Y."
+  [ev]
+  (.preventDefault ev)
+  (letfn [(move [e] (let [vh (-> (.-clientY e) (/ (.-innerHeight js/window)) (* 100) (max 10) (min 85))]
+                      (.setProperty (.. js/document -documentElement -style) "--top-h" (str vh "vh"))))
+          (up [_] (.removeEventListener js/document "pointermove" move)
+                  (.removeEventListener js/document "pointerup" up))]
+    (.addEventListener js/document "pointermove" move)
+    (.addEventListener js/document "pointerup" up)))
+
 (defn main-panel []
   (let [status @(rf/subscribe [::subs/status])
         draft? (some? @(rf/subscribe [::subs/draft-group]))]
@@ -331,6 +342,7 @@
      [:div.top
       [:div.video-pane [video-monitor] [frame-readout]]
       [:div.annot-pane (if draft? [annotation-form] [commentary])]]
+     [:div.divider-h {:on-pointer-down drag-top!}]
      [:div.timeline-pane
       [breadcrumbs]
       [toolbar]

tl/test/tl/scene_test.cljs

@@ -229,6 +229,34 @@
       (is (= {:seg :clip-a :f 0}   (:s (first rows))))
       (is (= {:seg :clip-a :f 100} (:e (first rows)))))))
 
+;; =========================================================================
+;; Suite 5 — a parent breaking a child's mark references
+;; =========================================================================
+
+(deftest broken-when-referenced-mark-deleted
+  (testing "deleting the referenced subclip dangles the child's mark"
+    (let [del (update-in x+y [:groups :ann-x :marks] #(vec (rest %)))]   ; drop s0
+      (is (= [:m/y0] (s/broken-marks del :ann-y)))
+      (is (= "referenced clip deleted" (s/broken-reason del :ann-y))))))
+
+(deftest broken-when-boundaries-shift-out-of-range
+  (testing "shrinking s0 below the child's :at makes the offset unreferenceable"
+    ;; y0 refs s0 at 10..20; trim s0 to length 15 so 20 is past its end
+    (let [trim (assoc-in x+y [:groups :ann-x :marks 0 :end] {:ref :clip-b :at 65})]
+      (is (= 15 (s/length (s/resolve-mark trim :ann-x (get-in trim [:groups :ann-x :marks 0])))))
+      (is (= [:m/y0] (s/broken-marks trim :ann-y)))
+      (is (= "reference trimmed away" (s/broken-reason trim :ann-y))))))
+
+(deftest broken-cascades-through-a-subclip
+  (testing "if the subclip's own ref breaks, the child referencing it breaks too"
+    (let [gone (update x+y :groups dissoc :clip-b)]    ; s0 refs clip-b, y0 refs s0
+      (is (= [:m/y0] (s/broken-marks gone :ann-y)))
+      (is (= "referenced clip deleted" (s/broken-reason gone :ann-y))))))
+
+(deftest healthy-annotation-has-no-reason
+  (testing "broken-reason is nil when every mark resolves"
+    (is (nil? (s/broken-reason x+y :ann-y)))))
+
 (deftest repeats-are-unambiguous
   (testing "two instances of A share a source frame but distinct locals (local is master)"
     (let [scene (with-group base :ann