Upd JS

static/script.js

@@ -16,14 +16,19 @@ function resetUI() {
       <div id="sampler-column"></div>
     </div>
   `;
+
+  // ✅ Update canvas & ctx properly
   const newCanvas = document.getElementById("lines-canvas");
-  ctx = canvas.getContext("2d");
   canvas.width = newCanvas.width = 0;
   canvas.height = newCanvas.height = 0;
+  ctx.clearRect?.(0, 0, canvas.width, canvas.height);
+
+  // 🧼 Remove old button & loader if exists
   if (loaderDiv) loaderDiv.remove();
   if (generateBtn) generateBtn.remove();
 }
 
+
 testerInput.addEventListener("change", handleFileChange);
 samplerInput.addEventListener("change", handleFileChange);