Streaming JSON in custom/* modules no longer renders any text. #4522

Issue

Streaming JSON in custom/* modules no longer renders text. Plain-text streaming is unaffected.

Any custom/* that runs a long-lived command emitting continuous JSON frames to stdout (with "return-type": "json" and "interval": 0) appears as an empty label on the bar. Polling / one-shot commands still render.

This also breaks common patterns like swaync-client -swb (see: SwayNotificationCenter Waybar example).

Notes

• Built-in modules render normally.
• Non-streaming custom/* (exec returns once per interval) render normally.
• Affected examples in my setup:
  • custom/cava (audio visualizer stream)
  • custom/notifications (e.g., swaync-client -swb)
• GTK widget tree shows the module label nodes (e.g., label#custom-cava.module), but the visible text is blank.
• Waybar logs show no JSON parse errors when the JSON is valid, and no other runtime errors.
• Replacing the stream with a polled one-shot (e.g., piping to head -n1 or adding a --once flag) restores visible output immediately.
• Plain-text streaming works (when return-type is omitted and raw text is formatted).
• JSON streaming fails only when return-type: "json" and interval: 0 (long-lived process).

Suspected regression window

Last known good: source build from late Sep 2025.
Broken after rebuilding from source on Oct 1–2, 2025.

Minimal Steps to Reproduce

1. Create a streaming generator (or use this swaync example):

   # ~/.config/waybar/scripts/fake_cava.py
   #!/usr/bin/env python3
   import time, itertools
   bars = ["▁","▂","▃","▄","▅","▆","▇","█"]
   for i in itertools.cycle(range(len(bars))):
       s = " ".join(bars[(i + j) % len(bars)] for j in range(10))
       print('{"text":"%s"}' % s, flush=True)
       time.sleep(0.5)

2. Use this Waybar config (streaming, interval: 0):

   {
     "layer": "top",
     "position": "top",
     "modules-left": ["custom/cava"],
     "custom/cava": {
       "exec": "python3 -u ~/.config/waybar/scripts/fake_cava.py",
       "return-type": "json",
       "interval": 0,
       "restart-interval": 0,
       "format": "{text}",
       "escape": true,
       "tooltip": false
     }
   }

3. Launch Waybar and observe: the custom/cava label exists in the widget tree, but no text is rendered.

Expected Result

Streaming custom/* modules that continuously emit valid JSON frames should render their text, as in previous versions.

Video

Debug Logs

waybar-trace.log

System Environment

Waybar: Waybar v0.14.0
Kernel: 6.16.8-200.fc42.x86_64
OS: "Fedora Linux 42 (KDE Plasma Desktop Edition)"
GTK: 3.24.49
gtkmm: 3.24.10
Wayland: 1.24.0
Hyprland: Hyprland 0.51.1 built from branch  at commit 71a1216abcc7031776630a6d88f105605c4dc1c9  ([gha] Nix: update inputs).
Date: Mon Sep 22 20:54:03 2025
Tag: v0.51.1, commits: 6436
built against:
 aquamarine 0.9.4
 hyprlang 0.6.4
 hyprutils 0.8.4
 hyprcursor 0.1.13
 hyprgraphics 0.1.6

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