fix(director): land on the requested shot

A shot action seeks to the target shot's startProgress, which
sceneTimingForShot computes as start / total. sceneSeekState multiplied
that back into seconds and compared it with each shot's end, but
(start / total) * total can round one ulp below start. The comparison
then put the seek at the very end of the previous shot. With five equal
3.9 s shots, "go to" shots 2, 3 and 5 landed on shots 1, 2 and 4.

Compare the requested progress against each shot's end / total instead,
the same division the caller used, so a shot's own start always selects
that shot. Exact boundaries still select the next shot, and anything
below a start still selects the previous one.

src/director/timeline.js

@@ -72,19 +72,27 @@ export function sceneSeekState(scene, progress, durationForShot, holdForShot) {
   let shotIndex = scene.shots.length - 1;
   for (let index = 0; index < scene.shots.length; index += 1) {
     const endElapsedSec = startElapsedSec + durations[index];
-    if (targetSec < endElapsedSec || index === scene.shots.length - 1) {
+    // Compare in progress units: a shot's startProgress (start / total)
+    // times total can round below its start and select the previous shot.
+    if (
+      normalized < endElapsedSec / totalSec ||
+      index === scene.shots.length - 1
+    ) {
       shotIndex = index;
       break;
     }
     startElapsedSec = endElapsedSec;
   }
+  // Keep the elapsed time inside the selected shot, so it agrees with
+  // shotIndex when targetSec rounded below the shot's start.
+  const elapsedSec = Math.max(targetSec, startElapsedSec);
   const shot = scene.shots[shotIndex];
   const flightDurationSec = shot.durationSec || DEFAULT_SHOT_DURATION_SEC;
   const holdDurationSec = holdForShot(scene, shot);
   const shotDurationSec = Math.max(0.001, flightDurationSec + holdDurationSec);
   const shotElapsedSec = Math.max(
     0,
-    Math.min(shotDurationSec, targetSec - startElapsedSec),
+    Math.min(shotDurationSec, elapsedSec - startElapsedSec),
   );
   const cameraProgress = clamp01(
     shotElapsedSec / Math.max(0.001, flightDurationSec),
@@ -99,8 +107,8 @@ export function sceneSeekState(scene, progress, durationForShot, holdForShot) {
   );
   const move = resolveCameraMove(scene, shot);
   return {
-    sceneProgress: totalSec > 0 ? targetSec / totalSec : 0,
-    sceneElapsedSec: targetSec,
+    sceneProgress: totalSec > 0 ? elapsedSec / totalSec : 0,
+    sceneElapsedSec: elapsedSec,
     sceneDurationSec: totalSec,
     shotIndex,
     shot,

src/director/timeline.test.mjs

@@ -49,3 +49,39 @@ test('camera seeking preserves cubic easing and shortest-angle orientation', ()
   );
   assert.deepEqual(cameraAtProgress(null, camera(20, 350), 0), camera(20, 350));
 });
+
+test('seeking to a shot start lands in that shot for every shot count and duration', () => {
+  const misses = [];
+  for (let count = 2; count <= 12; count += 1) {
+    for (let flightTenths = 1; flightTenths <= 100; flightTenths += 1) {
+      for (let holdTenths = 0; holdTenths <= 50; holdTenths += 1) {
+        const shots = Array.from({ length: count }, (_, index) => ({
+          id: `s${index}`,
+          // Every third shot is longer, so boundaries are not all equal steps.
+          durationSec: flightTenths / 10 + (index % 3 === 2 ? 1.7 : 0),
+          holdSec: holdTenths / 10,
+          camera: camera(20, 0),
+        }));
+        const mixed = { shots };
+        for (const [index, shot] of shots.entries()) {
+          const { startProgress, startElapsedSec } = sceneTimingForShot(
+            mixed,
+            shot,
+            duration,
+          );
+          const seek = sceneSeekState(mixed, startProgress, duration, hold);
+          // One or two ulps earlier still belongs to the previous shot.
+          const before = startProgress * (1 - Number.EPSILON);
+          const previous = sceneSeekState(mixed, before, duration, hold);
+          if (
+            seek.shotIndex !== index ||
+            seek.sceneElapsedSec < startElapsedSec ||
+            previous.shotIndex !== Math.max(0, index - 1)
+          )
+            misses.push(`${count}x${flightTenths}/${holdTenths}#${index}`);
+        }
+      }
+    }
+  }
+  assert.deepEqual(misses.slice(0, 5), []);
+});