fix: resolve crashes during video-only file loading

- Added null check for appState.vizData in sync.js to prevent TypeError during visualization reset.
- Implemented key validation guards in db.js manual offset functions to avoid IndexedDB DataErrors.
- Updated fileLoader.js to bypass manual offset lookup when no JSON filename is available.
- Added regression test 'tests/regression_video_only.test.js' to verify stable partial load paths.

steps/src/db.js

@@ -92,8 +92,8 @@ export function saveFileWithMetadata(key, file) {
 export function saveManualOffset(filename, offset) {
   return new Promise(async (resolve, reject) => {
     const database = await getDB();
-    if (!database) {
-      resolve(); // Fail silently if DB is not available
+    if (!database || !filename) {
+      resolve(); // Fail silently if DB is not available or filename is missing
       return;
     }
     const transaction = database.transaction(["manualOffsets"], "readwrite");
@@ -115,7 +115,7 @@ export function saveManualOffset(filename, offset) {
 export function loadManualOffset(filename) {
   return new Promise(async (resolve) => {
     const database = await getDB();
-    if (!database) {
+    if (!database || !filename) {
       resolve(null);
       return;
     }
@@ -142,7 +142,7 @@ export function loadManualOffset(filename) {
 export function deleteManualOffset(filename) {
   return new Promise(async (resolve) => {
     const database = await getDB();
-    if (!database) {
+    if (!database || !filename) {
       resolve();
       return;
     }

steps/src/fileLoader.js

@@ -401,10 +401,8 @@ async function calculateAndSetOffset() {
   }
 
   // 1. Try to load a manually saved offset for this specific file pair.
-  // We use the JSON filename as the primary key, but ideally, it should be a combo.
-  // For now, sticking to the user request: "if the user uploads a similarly named file".
-  // We'll use the JSON filename as the key.
-  const savedOffset = await loadManualOffset(appState.jsonFilename);
+  // We use the JSON filename as the primary key.
+  const savedOffset = appState.jsonFilename ? await loadManualOffset(appState.jsonFilename) : null;
 
   if (savedOffset !== null) {
     console.log(`Applying saved manual offset: ${savedOffset}ms`);

steps/src/sync.js

@@ -29,9 +29,18 @@ import { saveManualOffset } from "./db.js";
 export function resetVisualization() {
   appState.isPlaying = false;
   playPauseBtn.textContent = "Play";
+  
+  if (appState.vizData) {
     const numFrames = appState.vizData.radarFrames.length;
     timelineSlider.max = numFrames > 0 ? numFrames - 1 : 0;
     updateFrame(0, true); // Update to the first frame and force video seek
+  } else {
+    timelineSlider.max = 0;
+    timelineSlider.value = 0;
+    if (videoPlayer.src) {
+      videoPlayer.currentTime = 0;
+    }
+  }
 }
 
 // --- NEW Playback Control Functions ---

steps/tests/regression_video_only.test.js

@@ -0,0 +1,73 @@
+import { handleFiles } from "../src/fileLoader.js";
+import { appState } from "../src/state.js";
+import { initDB } from "../src/db.js";
+
+const resultsEl = document.getElementById('results');
+
+function test(description, testFunction) {
+    (async () => {
+        try {
+            await testFunction();
+            console.log(`✅ PASS: ${description}`);
+            resultsEl.innerHTML += `<p class="pass"><b>PASS:</b> ${description}</p>`;
+        } catch (error) {
+            console.error(`❌ FAIL: ${description}`, error);
+            resultsEl.innerHTML += `<p class="fail"><b>FAIL:</b> ${description}<br><pre>${error.stack || error}</pre></p>`;
+        }
+    })();
+}
+
+// Mocking dependencies for regression test
+async function setupMocks() {
+    return new Promise((resolve) => {
+        initDB(() => {
+            console.log("Test DB initialized");
+            resolve();
+        });
+    });
+}
+
+URL.createObjectURL = () => "blob:mock-video-url";
+URL.revokeObjectURL = () => {};
+
+window.p5 = class MockP5 {
+    constructor(sketch) { sketch(this); }
+    createCanvas() { return { parent: () => {} }; }
+    noLoop() {}
+    redraw() {}
+};
+
+test("Regression: handleFiles should not crash when loading only a video", async () => {
+    // 1. Setup - clear vizData
+    appState.vizData = null;
+    appState.videoFilename = "";
+    
+    const videoPlayer = document.getElementById('video-player');
+    const mockVideoFile = new File(['fake video'], "test.mp4", { type: "video/mp4" });
+
+    // Mock video events
+    const triggerEvents = () => {
+        setTimeout(() => {
+            videoPlayer.dispatchEvent(new Event('loadedmetadata'));
+            videoPlayer.dispatchEvent(new Event('canplaythrough'));
+        }, 10);
+    };
+
+    const observer = new MutationObserver(triggerEvents);
+    observer.observe(videoPlayer, { attributes: true, attributeFilter: ['src'] });
+
+    // 2. Execution
+    try {
+        await handleFiles([mockVideoFile]);
+    } catch (e) {
+        throw new Error(`Crash detected during video-only load: ${e.message}`);
+    }
+
+    // 3. Verification
+    await new Promise(resolve => setTimeout(resolve, 100));
+    observer.disconnect();
+
+    if (appState.videoFilename !== "test.mp4") {
+        throw new Error("Video filename not set correctly in appState");
+    }
+});