Lay root clips at timeline positions, not source positions

The root timeline placed each clip at its media-in (source) frame, which is
fractional (media-offset 50641.358…), so adjacent clips of a contiguous multicam
edit showed ~1-frame gaps; clicking a gap hit no segment and local->source
returned the last frame ("glitch to the end"). The OTIO timeline is gapless —
clips tile by their accumulated timeline :start. Carry :start through from-otio
and lay content-segments at [start, start+len] (src stays the source range), so
the assembled program is contiguous and maps cleanly to scattered source frames.

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

tl/src/tl/scene.cljs

@@ -170,15 +170,19 @@
   "The clip-segments that make up context `ctx` — what you draw and select
    against. An annotation's marks reference clips, so that's just `resolve`. The
    root timeline doesn't enumerate its clips (they're a parentless pool), so
-   there it's the pool laid in root's identity coordinate (local = source)."
+   there it's the pool laid at each clip's TIMELINE position (:start) with :src =
+   its source range — so the assembled program tiles contiguously even though
+   the underlying source frames are scattered (and fractional)."
   [scene ctx]
   (if (= :timeline (:type (grp scene ctx)))
     (->> (:groups scene)
          (keep (fn [[gid g]]
                  (when (= :clip (:type g))
-                   (let [seg (first (resolve scene gid))]
+                   (let [seg (first (resolve scene gid))
-                     (assoc seg :mark gid :local (:src seg))))))   ; ref clip by group id
+                         [sa sb] (:src seg)
-         (sort-by (comp first :src))
+                         st (:start g 0)]
+                     (assoc seg :mark gid :local [st (+ st (- sb sa))])))))
+         (sort-by (comp first :local))
          vec)
     (resolve scene ctx)))
 
@@ -278,6 +282,7 @@
         clips     (into {} (for [t vtracks c (:clips t)]
                              [(keyword (:id c))
                               {:type :clip :parent nil :name (:name c)
+                               :start (:start c)              ; timeline position (frames)
                                :marks [{:id    (keyword (str (:id c) "-m"))
                                         :start (:media-in c)
                                         :end   (+ (:media-in c) (:duration c))

tl/test/tl/scene_test.cljs

@@ -8,9 +8,10 @@
 (def base
   {:tracks {:t0 {:name "A-roll"} :t1 {:name "B-roll"} :t2 {:name "C-roll"}}
    :groups {:root   {:type :timeline :parent nil :marks [{:id :m/root :start 0 :end 300}]}
-            :clip-a {:type :clip :parent nil :marks [{:id :m/a :start 0   :end 100 :track :t0}]}
+            ;; :start = timeline position; here it equals source so root is identity
-            :clip-b {:type :clip :parent nil :marks [{:id :m/b :start 100 :end 200 :track :t1}]}
+            :clip-a {:type :clip :parent nil :start 0   :marks [{:id :m/a :start 0   :end 100 :track :t0}]}
-            :clip-c {:type :clip :parent nil :marks [{:id :m/c :start 200 :end 300 :track :t2}]}}})
+            :clip-b {:type :clip :parent nil :start 100 :marks [{:id :m/b :start 100 :end 200 :track :t1}]}
+            :clip-c {:type :clip :parent nil :start 200 :marks [{:id :m/c :start 200 :end 300 :track :t2}]}}})
 
 (defn with-group [scene gid g] (assoc-in scene [:groups gid] g))
 (defn refm [id clip a b] {:id id :start {:ref clip :at a} :end {:ref clip :at b}})
@@ -177,10 +178,12 @@
 
 (deftest seed-from-otio
   (testing "from-otio seeds tracks + clip mark-groups + root; content-segments at root tiles the clips"
+    ;; two clips cut back to back on the timeline (:start 0 then 100), like a
+    ;; multicam program, even though their source frames aren't contiguous
     (let [parsed {:fps 24 :duration 200
-                  :tracks [{:index 0 :kind :video :name "Wide" :clips [{:id "t0-c0" :name "a" :media-in 0   :duration 100}]}
+                  :tracks [{:index 0 :kind :video :name "Wide" :clips [{:id "t0-c0" :name "a" :start 0   :media-in 0   :duration 100}]}
-                           {:index 1 :kind :video :name "CU"   :clips [{:id "t1-c0" :name "b" :media-in 100 :duration 100}]}
+                           {:index 1 :kind :video :name "CU"   :clips [{:id "t1-c0" :name "b" :start 100 :media-in 500 :duration 100}]}
-                           {:index 2 :kind :audio :name "Aud"  :clips [{:id "t2-c0" :name "x" :media-in 0   :duration 50}]}]}
+                           {:index 2 :kind :audio :name "Aud"  :clips [{:id "t2-c0" :name "x" :start 0   :media-in 0   :duration 50}]}]}
           scene  (s/from-otio parsed)]
       (is (= 2 (count (:tracks scene))))                    ; audio track dropped
       (is (contains? (:groups scene) :t0-c0))