Replace server-side slot-visibility updates with client-side JS

Gradio 5.25's client rejects gr.Group visibility outputs with "Too many
arguments provided for the endpoint" (500 on /queue/join), even when
isolated in their own .then() step — confirmed via fn_index tracing
that this wasn't session staleness but a real, reproducible bug tied
specifically to Group components as event outputs.

Toggling display on the slot Group's DOM element (by elem_id) via a
fn=None/js= event needs no server round-trip, so there's nothing left
for that validation to reject.

app.py

@@ -2474,7 +2474,7 @@ def _make_output_slots(tab_prefix: str) -> tuple:
     grps, vids, waveforms = [], [], []
     for i in range(MAX_SLOTS):
         slot_id = f"{tab_prefix}_{i}"
-        with gr.Group(visible=(i == 0)) as g:
+        with gr.Group(visible=(i == 0), elem_id=f"slot_grp_{slot_id}") as g:
             vids.append(gr.Video(label=f"Generation {i+1} — Video",
                                  elem_id=f"slot_vid_{slot_id}",
                                  show_download_button=False))
@@ -2532,11 +2532,6 @@ def _on_video_upload_taro(video_file, num_steps, crossfade_s):
     return gr.update(maximum=max_s, value=min(1, max_s))
 
 
-def _update_slot_visibility(n):
-    n = int(n)
-    return [gr.update(visible=(i < n)) for i in range(MAX_SLOTS)]
-
-
 def _btn_generating():
     """Disable a Generate button and show a busy label. Chained as the first
     .click() step so it fires immediately, before the (possibly slow) generate
@@ -2586,6 +2581,22 @@ _GLOBAL_JS = """
   if (window._wf_global_listener) return;  // already registered
   window._wf_global_listener = true;
 
+  // Toggle output-slot visibility by directly flipping display on the slot
+  // Group's DOM element. This used to be a server round-trip (a .then() that
+  // returned gr.update(visible=...) for each Group), but Gradio 5.25's client
+  // rejects Group-visibility outputs with "Too many arguments provided for
+  // the endpoint" (500 on /queue/join) even when isolated in their own step —
+  // doing it client-side sidesteps that bug entirely, no server call needed.
+  window._updateSlotVisibility = function(prefix, n) {
+    let i = 0;
+    while (true) {
+      const el = document.getElementById('slot_grp_' + prefix + '_' + i);
+      if (!el) break;
+      el.style.display = (i < n) ? '' : 'none';
+      i++;
+    }
+  };
+
   // ── ZeroGPU quota attribution ──
   // HF Spaces run inside an iframe on huggingface.co. Gradio's own JS client
   // gets ZeroGPU auth headers (x-zerogpu-token, x-zerogpu-uuid) by sending a
@@ -3017,7 +3028,7 @@ with gr.Blocks(title="Generate Audio for Video", css=_SLOT_CSS, js=_GLOBAL_JS) a
                     taro_btn        = gr.Button("Generate", variant="primary")
 
                 with gr.Column(scale=1):
-                    (taro_slot_grps, taro_slot_vids,
+                    (_, taro_slot_vids,
                      taro_slot_waves) = _make_output_slots("taro")
 
             # Hidden regen plumbing — render=False so no DOM element is created,
@@ -3034,9 +3045,10 @@ with gr.Blocks(title="Generate Audio for Video", css=_SLOT_CSS, js=_GLOBAL_JS) a
                     outputs=[taro_samples],
                 )
             taro_samples.change(
-                fn=_update_slot_visibility,
+                fn=None,
                 inputs=[taro_samples],
-                outputs=taro_slot_grps,
+                outputs=[],
+                js="(n) => window._updateSlotVisibility('taro', n)",
             )
 
             def _run_taro(video, seed, cfg, steps, mode, cf_dur, cf_db, n):
@@ -3065,9 +3077,10 @@ with gr.Blocks(title="Generate Audio for Video", css=_SLOT_CSS, js=_GLOBAL_JS) a
                         taro_cf_dur, taro_cf_db, taro_samples],
                 outputs=taro_slot_vids + taro_slot_waves,
             ).then(
-                fn=_update_slot_visibility,
+                fn=None,
                 inputs=[taro_samples],
-                outputs=taro_slot_grps,
+                outputs=[],
+                js="(n) => window._updateSlotVisibility('taro', n)",
             ).then(
                 fn=_btn_ready,
                 inputs=None,
@@ -3101,7 +3114,7 @@ with gr.Blocks(title="Generate Audio for Video", css=_SLOT_CSS, js=_GLOBAL_JS) a
                     mma_btn         = gr.Button("Generate", variant="primary")
 
                 with gr.Column(scale=1):
-                    (mma_slot_grps, mma_slot_vids,
+                    (_, mma_slot_vids,
                      mma_slot_waves) = _make_output_slots("mma")
 
             # Hidden regen plumbing — render=False so no DOM element is created,
@@ -3110,9 +3123,10 @@ with gr.Blocks(title="Generate Audio for Video", css=_SLOT_CSS, js=_GLOBAL_JS) a
             mma_regen_state = gr.Textbox(value="", render=False)
 
             mma_samples.change(
-                fn=_update_slot_visibility,
+                fn=None,
                 inputs=[mma_samples],
-                outputs=mma_slot_grps,
+                outputs=[],
+                js="(n) => window._updateSlotVisibility('mma', n)",
             )
 
             def _run_mmaudio(video, prompt, neg, seed, cfg, steps, cf_dur, cf_db, n):
@@ -3129,9 +3143,10 @@ with gr.Blocks(title="Generate Audio for Video", css=_SLOT_CSS, js=_GLOBAL_JS) a
                         mma_cfg, mma_steps, mma_cf_dur, mma_cf_db, mma_samples],
                 outputs=mma_slot_vids + mma_slot_waves,
             ).then(
-                fn=_update_slot_visibility,
+                fn=None,
                 inputs=[mma_samples],
-                outputs=mma_slot_grps,
+                outputs=[],
+                js="(n) => window._updateSlotVisibility('mma', n)",
             ).then(
                 fn=_btn_ready,
                 inputs=None,
@@ -3164,7 +3179,7 @@ with gr.Blocks(title="Generate Audio for Video", css=_SLOT_CSS, js=_GLOBAL_JS) a
                     hf_btn          = gr.Button("Generate", variant="primary")
 
                 with gr.Column(scale=1):
-                    (hf_slot_grps, hf_slot_vids,
+                    (_, hf_slot_vids,
                      hf_slot_waves) = _make_output_slots("hf")
 
             # Hidden regen plumbing — render=False so no DOM element is created,
@@ -3173,9 +3188,10 @@ with gr.Blocks(title="Generate Audio for Video", css=_SLOT_CSS, js=_GLOBAL_JS) a
             hf_regen_state  = gr.Textbox(value="", render=False)
 
             hf_samples.change(
-                fn=_update_slot_visibility,
+                fn=None,
                 inputs=[hf_samples],
-                outputs=hf_slot_grps,
+                outputs=[],
+                js="(n) => window._updateSlotVisibility('hf', n)",
             )
 
             def _run_hunyuan(video, prompt, neg, seed, guidance, steps, size, cf_dur, cf_db, n):
@@ -3192,9 +3208,10 @@ with gr.Blocks(title="Generate Audio for Video", css=_SLOT_CSS, js=_GLOBAL_JS) a
                         hf_guidance, hf_steps, hf_size, hf_cf_dur, hf_cf_db, hf_samples],
                 outputs=hf_slot_vids + hf_slot_waves,
             ).then(
-                fn=_update_slot_visibility,
+                fn=None,
                 inputs=[hf_samples],
-                outputs=hf_slot_grps,
+                outputs=[],
+                js="(n) => window._updateSlotVisibility('hf', n)",
             ).then(
                 fn=_btn_ready,
                 inputs=None,