Stop resampling the extend start frame every hop

upload() ran fitStill (ffmpeg scale+pad) on every image except hero,
including the start/handoff still. Four chained extends resampled the
anchor frame four times and destroyed identity even with the hero
reference and lock text intact.

- upload(): only fit when the still's aspect truly differs from the
  job canvas (needsFit); never fit hero or start regardless.
- Track handoffSource (anchor|extract) and whether the start frame was
  fitted; surface both in job metadata and Details.
- extendSelected(): re-binding an already-selected clip on task switch
  must not stomp the user's overridden locks/settings back to the
  clip's saved values (regression introduced by the last hotfix push).
- Add tests/studio2-extend-fit.test.mjs: two extends at identical
  width/height reach Comfy with the anchor PNG byte-identical, no
  ffmpeg fit.

Co-authored-by: Cursor <cursoragent@cursor.com>

server/utils/studio2/media.ts

@@ -2,6 +2,7 @@ import { mkdtempSync, writeFileSync, readFileSync, rmSync } from 'node:fs'
 import { join } from 'node:path'
 import { tmpdir } from 'node:os'
 import { runFfmpeg } from '../ffmpeg'
+import { imageDimensions } from '../resolution'
 export async function fitStill(data: Buffer, width: number, height: number) {
   const dir=mkdtempSync(join(tmpdir(),'studio2-fit-'))
   try {
@@ -11,3 +12,12 @@ export async function fitStill(data: Buffer, width: number, height: number) {
     return readFileSync(output)
   } finally { rmSync(dir,{recursive:true,force:true}) }
 }
+/** Only scale/pad when the still's aspect truly differs from the job canvas.
+ * A matching frame (typical extend handoff) must reach Comfy byte-identical. */
+export function needsFit(data: Buffer, width: number, height: number) {
+  const size = imageDimensions(data)
+  if (!size?.width || !size?.height) return true
+  if (size.width === width && size.height === height) return false
+  const target = width / height, actual = size.width / size.height
+  return Math.abs(target - actual) > 0.01
+}

server/utils/studio2/runner.ts

@@ -1,7 +1,7 @@
 import { stylePrompt } from '~/shared/studio2/styles.mjs';
 import { purge } from './cleanup';
 import { watchProgress } from './progress';
-import { fitStill } from './media';
+import { fitStill, needsFit } from './media';
 import { resolveRequestSize } from './size';
 import { queueSeeds } from '~/shared/studio2/seed.mjs';
 import { readFileSync, mkdirSync, existsSync, unlinkSync } from 'node:fs';
@@ -29,9 +29,18 @@ type DiskFile = {
     type: string;
 };
 function update(r: any, state: string) { r.state = state; r.updatedAt = Date.now(); saveRecord(r); }
+// Labels whose pixels must never be resampled: the hero identity still, and the
+// extend/video start frame (which is either the exact anchor PNG or an already-canvas-sized
+// extracted last frame). Fitting these every hop is what destroys identity across chained extends.
+const NEVER_FIT = new Set(['hero', 'start']);
 async function upload(r: any, name: string, data: Buffer): Promise<string> {
     const prefix = String(useRuntimeConfig().comfyFilenamePrefix).replace(/\/$/, '') + `/studio2/${r.id}/${r.index}`;
-    if (name !== 'hero') data = await fitStill(data, r.request.settings.width, r.request.settings.height);
+    let fitted = false;
+    if (!NEVER_FIT.has(name) && needsFit(data, r.request.settings.width, r.request.settings.height)) {
+        data = await fitStill(data, r.request.settings.width, r.request.settings.height);
+        fitted = true;
+    }
+    if (name === 'start') { r.handoffFitted = fitted; saveRecord(r); }
     const body = new FormData();
     body.append('image', new Blob([new Uint8Array(data)]), name + '.png');
     body.append('subfolder', prefix);
@@ -71,7 +80,8 @@ async function prepareGraph(r: any) {
             const dir = join(studio2Root(), r.id);
             mkdirSync(dir, { recursive: true });
             const dest = join(dir, 'handoff.png');
-            await resolveExtensionHandoffFrame({ ownerKey: r.owner, sourceClipId: q.startClipId, sourceVideoPath: clipVideoPath(r.owner, q.startClipId), destPath: dest });
+            const handoff = await resolveExtensionHandoffFrame({ ownerKey: r.owner, sourceClipId: q.startClipId, sourceVideoPath: clipVideoPath(r.owner, q.startClipId), destPath: dest });
+            r.handoffSource = handoff.source;
             start = await upload(r, 'start', readFileSync(dest));
             unlinkSync(dest);
         }
@@ -208,7 +218,7 @@ async function run(r: any, job: Job) {
             saveRecord(r);
             r.savedAt = Date.now();
             r.wallTime = (r.savedAt - r.startedAt) / 1000;
-            const metadata = { ...structuredClone(q), id: r.id, sourceStillId:r.sourceStillId,heroReferenceAttached:r.heroReferenceAttached,sampleLatent:r.sampleLatent,sampleDenoise:r.sampleDenoise,kind: video ? 'video' : 'image', graphId: r.graphId, promptId: r.promptId, queuedAt: r.queuedAt, startedAt: r.startedAt, savedAt: r.savedAt, gpuSeconds: r.gpuSeconds, wallTime: r.wallTime, outputWidth: asset.width || s.width, outputHeight: asset.height || s.height, purgeResult: 'Left on host' };
+            const metadata = { ...structuredClone(q), id: r.id, sourceStillId:r.sourceStillId,heroReferenceAttached:r.heroReferenceAttached,sampleLatent:r.sampleLatent,sampleDenoise:r.sampleDenoise,kind: video ? 'video' : 'image', graphId: r.graphId, promptId: r.promptId, queuedAt: r.queuedAt, startedAt: r.startedAt, savedAt: r.savedAt, gpuSeconds: r.gpuSeconds, wallTime: r.wallTime, outputWidth: asset.width || s.width, outputHeight: asset.height || s.height, purgeResult: 'Left on host', handoffSource: r.handoffSource || null, fitted: video && q.startClipId ? !!r.handoffFitted : null };
             await attachStudio2Metadata(r.owner, asset.id, metadata);
             if (!r.outputs.some((o:any)=>o.id===asset.id)) r.outputs.push({id:asset.id,kind:video?'clip':'still',studio2:metadata});
             saveRecord(r);