Fix regen: single-event trigger + generator-safe early-exit

Two bugs causing regeneration to never reach Python:

1. clear-then-set fired TWO .change() events. The first (empty string)
caused the generator to hit a bare `return` — but Python generators
with a bare return raise StopIteration immediately, which Gradio 5
SSR interprets as an SSE stream error. This errored state blocked
the second (real value) call from completing.
Fix: remove the clear step entirely; use a timestamp suffix in the
trigger value so repeat clicks always produce a unique string and
React's change detection always fires.

2. Early-exit paths used `return tuple` inside a generator function.
In Python, any function containing `yield` is always a generator —
`return value` inside a generator is silently dropped (only
StopIteration is raised). Gradio 5 SSR then sees an empty stream.
Fix: change all early-exit paths to `yield ...; return` so Gradio
receives a proper final update before the stream closes.

Trigger format is now: "slot_id|seg_idx|timestamp|{stateJSON}"
All three handlers (TARO/MMA/HF) updated accordingly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app.py

@@ -1568,7 +1568,10 @@ _GLOBAL_JS = """
     }
 
     // Use native setter to bypass React's controlled-input tracking.
-    // Clear to '' first so a repeat click on the same segment still fires .change().
+    // We do NOT clear to '' first — that would fire a spurious .change() event
+    // which (as a generator function) causes an SSE stream error that blocks
+    // the real call. Instead we use a timestamp suffix to ensure uniqueness
+    // so repeat clicks on the same segment always look like a new value.
     const desc = Object.getOwnPropertyDescriptor(HTMLInputElement.prototype, 'value')
               || Object.getOwnPropertyDescriptor(HTMLTextAreaElement.prototype, 'value');
     function setNative(val) {
@@ -1577,13 +1580,10 @@ _GLOBAL_JS = """
       input.dispatchEvent(new Event('input',  {bubbles: true}));
       input.dispatchEvent(new Event('change', {bubbles: true}));
     }
-    // Encode: "slot_id|seg_idx|{stateJSON}"
-    const triggerVal = slot_id + '|' + idx + '|' + stateJson;
-    setNative('');
-    setTimeout(function() {
-      setNative(triggerVal);
-      console.log('[fireRegen] fired trigger for', slot_id, 'seg', idx);
-    }, 50);
+    // Encode: "slot_id|seg_idx|ts|{stateJSON}"  (ts = timestamp for uniqueness)
+    const triggerVal = slot_id + '|' + idx + '|' + Date.now() + '|' + stateJson;
+    setNative(triggerVal);
+    console.log('[fireRegen] fired trigger for', slot_id, 'seg', idx);
 
     const lbl = document.getElementById('wf_seglabel_' + slot_id);
     if (lbl) lbl.textContent = 'Regenerating Seg ' + (idx + 1) + '...';
@@ -1689,14 +1689,14 @@ with gr.Blocks(title="Generate Audio for Video", css=_SLOT_CSS, js=_GLOBAL_JS) a
                         print(f"[regen TARO] trigger_val_len={len(trigger_val) if trigger_val else 0} video={video!r}")
                         if not trigger_val:
                             print(f"[regen TARO] early-exit: trigger_val empty")
-                            return gr.update(), gr.update(), gr.update(value="")
-                        # Trigger format: "slot_id|seg_idx|{stateJSON}"
-                        parts = trigger_val.split("|", 2)
-                        if len(parts) != 3 or parts[0] != _sid:
-                            print(f"[regen TARO] early-exit: parts[0]={parts[0]!r} expected={_sid!r}")
-                            return gr.update(), gr.update(), gr.update(value="")
+                            yield gr.update(), gr.update(), gr.update(); return
+                        # Trigger format: "slot_id|seg_idx|timestamp|{stateJSON}"
+                        parts = trigger_val.split("|", 3)
+                        if len(parts) != 4 or parts[0] != _sid:
+                            print(f"[regen TARO] early-exit: parts={parts[:2]} expected slot={_sid!r}")
+                            yield gr.update(), gr.update(), gr.update(); return
                         seg_idx    = int(parts[1])
-                        state_json = parts[2]
+                        state_json = parts[3]
                         print(f"[regen TARO] slot={_sid} seg_idx={seg_idx} state_json_len={len(state_json)}")
                         lock = _get_slot_lock(_sid)
                         with lock:
@@ -1781,14 +1781,14 @@ with gr.Blocks(title="Generate Audio for Video", css=_SLOT_CSS, js=_GLOBAL_JS) a
                         print(f"[regen MMA] trigger_val_len={len(trigger_val) if trigger_val else 0} video={video!r}")
                         if not trigger_val:
                             print(f"[regen MMA] early-exit: trigger_val empty")
-                            return gr.update(), gr.update(), gr.update(value="")
-                        # Trigger format: "slot_id|seg_idx|{stateJSON}"
-                        parts = trigger_val.split("|", 2)
-                        if len(parts) != 3 or parts[0] != _sid:
-                            print(f"[regen MMA] early-exit: parts[0]={parts[0]!r} expected={_sid!r}")
-                            return gr.update(), gr.update(), gr.update(value="")
+                            yield gr.update(), gr.update(), gr.update(); return
+                        # Trigger format: "slot_id|seg_idx|timestamp|{stateJSON}"
+                        parts = trigger_val.split("|", 3)
+                        if len(parts) != 4 or parts[0] != _sid:
+                            print(f"[regen MMA] early-exit: parts={parts[:2]} expected slot={_sid!r}")
+                            yield gr.update(), gr.update(), gr.update(); return
                         seg_idx    = int(parts[1])
-                        state_json = parts[2]
+                        state_json = parts[3]
                         print(f"[regen MMA] slot={_sid} seg_idx={seg_idx} state_json_len={len(state_json)}")
                         lock = _get_slot_lock(_sid)
                         with lock:
@@ -1874,14 +1874,14 @@ with gr.Blocks(title="Generate Audio for Video", css=_SLOT_CSS, js=_GLOBAL_JS) a
                         print(f"[regen HF] trigger_val_len={len(trigger_val) if trigger_val else 0} video={video!r}")
                         if not trigger_val:
                             print(f"[regen HF] early-exit: trigger_val empty")
-                            return gr.update(), gr.update(), gr.update(value="")
-                        # Trigger format: "slot_id|seg_idx|{stateJSON}"
-                        parts = trigger_val.split("|", 2)
-                        if len(parts) != 3 or parts[0] != _sid:
-                            print(f"[regen HF] early-exit: parts[0]={parts[0]!r} expected={_sid!r}")
-                            return gr.update(), gr.update(), gr.update(value="")
+                            yield gr.update(), gr.update(), gr.update(); return
+                        # Trigger format: "slot_id|seg_idx|timestamp|{stateJSON}"
+                        parts = trigger_val.split("|", 3)
+                        if len(parts) != 4 or parts[0] != _sid:
+                            print(f"[regen HF] early-exit: parts={parts[:2]} expected slot={_sid!r}")
+                            yield gr.update(), gr.update(), gr.update(); return
                         seg_idx    = int(parts[1])
-                        state_json = parts[2]
+                        state_json = parts[3]
                         print(f"[regen HF] slot={_sid} seg_idx={seg_idx} state_json_len={len(state_json)}")
                         lock = _get_slot_lock(_sid)
                         with lock: