feat(osr): smooth, never-stuck tile resize via begin-frame pump + GPU blit#8
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… blit Reworks the OSR present/resize path so webview tiles resize like a native browser instead of freezing or stuttering. - Own the frame clock: window_info.external_begin_frame_enabled + a per-slot SendExternalBeginFrame pump. CEF's internal damage-driven scheduler could skip the post-resize frame (a resize is viewport-only damage on an idle page), leaving the tile stuck at the old size until a scroll forced a tick. Driving begin-frames ourselves makes every resize produce a frame. The pump idles to a slow poll while the tile is hidden and dies when it's disposed. - Resize flow-control + watchdog (CefWebSession): keep one resize in flight, send the next when its present promotes, so the reflow paces to cef_host's actual rate instead of racing ahead and stranding presents on superseded surface ids (which froze the texture mid-drag). A generation-tagged watchdog re-kicks a wedged step and, worst case, force-shows the already-painted surface -- no permanent stuck on a static page's single post-resize frame. - Pending-surface promotion: serve the old surface to Flutter until cef_host paints the new one, so a resize no longer flashes blank. - GPU Metal blit present: copy CEF's accelerated IOSurface into the host surface with a Metal blit instead of a CPU IOSurfaceLock+memcpy. Keeps frame data on the GPU end-to-end (no GPU->CPU readback) -- a real win on discrete-GPU / Windows / Linux, ~neutral on unified-memory Apple Silicon, and the contract-mandated cross-platform shape (see the PORTING note on CompositeMetalLocked). Popups and the software-OSR OnPaint fallback stay on the CPU path; falls back to CPU if Metal is unavailable. - Dart resizes on every layout change now that the native side paces. True zero-copy (handing CEF's surface straight to Flutter) is impossible: CEF's pool contract reclaims the accelerated surface at callback return, so a copy into a client-owned texture inside the callback is mandated on every platform. This makes that copy a GPU blit. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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What
Makes OSR webview tiles resize like a native browser instead of freezing or stuttering. Three problems, fixed in the present/resize path:
external_begin_frame_enabled+ a per-slotSendExternalBeginFramepump (idles while hidden, dies on dispose).Plus: the per-frame present copy is now a GPU Metal blit instead of a CPU
IOSurfaceLock+memcpy — keeps frame data on-GPU end-to-end (no readback). Popups and the software-OSR fallback stay CPU.Why not true zero-copy
CEF's pool contract (
cef_render_handler.h) reclaims the accelerated surface at callback return, so handing it straight to Flutter is impossible — a copy into a client-owned texture inside the callback is mandated on every platform. So we make that copy a GPU blit, which is also the contract-mandated cross-platform shape (Windows D3D11 shared-handle, Linux dmabuf).Test
Verified in the example app on M4 Pro: loads, resizes smoothly with no stuck/flash on flutter.dev (static) and YouTube (video),
<select>dropdown still correct (CPU path), GPU-blit path confirmed active via the host log.🤖 Generated with Claude Code