diff --git a/services/web-ui/public/js/timeline.js b/services/web-ui/public/js/timeline.js
index 61d30f1..4d0bcf8 100644
--- a/services/web-ui/public/js/timeline.js
+++ b/services/web-ui/public/js/timeline.js
@@ -337,14 +337,14 @@
     var origTlOut   = clip.timeline_out_frames;
     var origSrcIn   = clip.source_in_frames;
     var origSrcOut  = clip.source_out_frames;
-    // Maximum source out is the asset's full duration in frames
+    // Full asset duration in frames (null when unknown)
     var assetFrames = clip.duration_ms
       ? Math.round((clip.duration_ms / 1000) * s.fps)
-      : origSrcOut;
+      : null;
 
     function onMove(ev) {
-      var dx     = ev.clientX - startX;
-      var dFr    = pxToFrames(dx);
+      var dx  = ev.clientX - startX;
+      var dFr = pxToFrames(dx);
       if (side === 'left') {
         var newTlIn = Math.max(0, Math.min(origTlIn + dFr, origTlOut - 1));
         var dIn     = newTlIn - origTlIn;
@@ -353,8 +353,15 @@
       } else {
         var newTlOut = Math.max(origTlIn + 1, origTlOut + dFr);
         var dOut     = newTlOut - origTlOut;
-        clip.timeline_out_frames = newTlOut;
-        clip.source_out_frames   = Math.min(assetFrames, origSrcOut + dOut);
+        // When we know the asset duration, don't extend past the source boundary
+        if (assetFrames !== null) {
+          var maxOut = assetFrames - origSrcOut;
+          if (dOut > maxOut) dOut = maxOut;
+        }
+        clip.timeline_out_frames = origTlOut + dOut;
+        clip.source_out_frames   = assetFrames !== null
+          ? Math.min(assetFrames, origSrcOut + dOut)
+          : origSrcOut + dOut;
       }
       _renderClips();
     }