feat(vegas): render plugin content off the render thread, and keep it off the GIL when the panel needs it (#630)

DisplayManager.offscreen() gives a thread its own canvas, so Vegas renders every plugin's ticker content on its prefetch thread instead of pausing the scroll for canvas-bound plugins on the render thread. A render gate (src/common/render_gate.py, vegas_scroll.prefetch_gate, on by default with the GIL-releasing binding) lets the prefetch thread run Python only while the render thread waits in SwapOnVSync: on hdpi, frames 2+ refreshes late fell eightfold and late frames overall from 0.90% to 0.60%. See docs/OFFSCREEN_RENDERING.md.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/vegas_mode/render_pipeline.py

@@ -10,6 +10,7 @@ import os
 import time
 import threading
 from collections import deque
+from contextlib import nullcontext
 from typing import Optional, List, Any, Dict, Deque
 from PIL import Image
 
@@ -383,8 +384,12 @@ class RenderPipeline:
                     os.nice(10)
                 except (OSError, AttributeError):
                     pass
+                # With vegas_scroll.prefetch_gate on, run only while the render
+                # thread waits on vsync; see src/common/render_gate.py.
+                gate = getattr(self.display_manager, 'render_gate', None)
                 try:
-                    group = self.stream_manager.take_next_group(offscreen_only=True)
+                    with gate.yielding() if gate is not None else nullcontext():
+                        group = self.stream_manager.take_next_group(offscreen_only=True)
                 except Exception:
                     logger.exception("Background prefetch failed")
                     group = []
@@ -509,9 +514,12 @@ class RenderPipeline:
             grouped = [(pid, imgs) for pid, imgs in grouped if imgs]
 
             if not grouped:
-                # Everything in this group is queued; the queue will extend the
-                # strip as it drains, so this is not a failure.
-                logger.info("Whole group deferred; strip will extend as it drains")
+                if deferred:
+                    # Everything in this group is queued; the queue will extend
+                    # the strip as it drains, so this is not a failure.
+                    logger.info("Whole group deferred; strip will extend as it drains")
+                else:
+                    logger.info("Nothing to show in this group; fetching the next")
                 self.start_prefetch()
                 return bool(deferred)