Reworked threading model.

[bkaradzic-microsoft]

[Updated by Copilot on behalf of @bghgary]

Context

Replaces the old per-thread encoder model (GetEncoderForThread/EndEncoders + UpdateToken wrapping SafeTimespanGuarantor) with a single frame encoder owned by DeviceImpl, synchronized by a FrameCompletionScope RAII gate. SafeTimespanGuarantor, UpdateToken, and the Update classes are removed. DeviceUpdate remains as a no-op shim — removing it is an API break and a separate PR (#1653).

Model

StartRenderingCurrentFrame acquires the encoder (bgfx::begin) and opens the gate; FinishRenderingCurrentFrame waits for all FrameCompletionScopes to release, closes the gate, ends the encoder, and submits the frame. All consumers (NativeEngine, Canvas, NativeXr) read the shared encoder via GetActiveEncoder(). SubmitCommands and ReadTexture each hold a stack-scoped FrameCompletionScope, so the encoder can't be ended mid-work; when invoked outside a frame (e.g. an XHR callback) the scope blocks until the next frame opens.

Main Thread                       JS Thread                 bgfx render thread
  StartRenderingCurrentFrame()
    m_frameEncoder = bgfx::begin(true)
    m_frameBlocked = false; CV notify
    tick frameStartDispatcher  -----> RAF callbacks
                                        submitCommands()
                                        AcquireFrameCompletionScope (immediate; frame open)
                                        GetEncoder() -> m_frameEncoder; draw...
                                        scope released; pendingScopes-- -> CV notify
  FinishRenderingCurrentFrame()
    wait scopes == 0; m_frameBlocked = true
    tick beforeRenderDispatcher
    bgfx::end(m_frameEncoder)
    Frame() -> bgfx::frame()  ------------------------------> GPU submit & render
    tick afterRenderDispatcher

Synchronization rules

1. Single encoder, acquired before the gate opens (mutex gives memory ordering) and ended only after all scopes release.
2. SubmitCommands/ReadTexture always hold a scope — the encoder can't be ended mid-command, regardless of microtask draining or reentrancy.
3. Apps must bracket any main-thread wait for JS work with Start/Finish, or SubmitCommands blocks forever waiting for the gate (HeadlessScreenshotApp, StyleTransferApp, UnitTests do this).
4. Canvas::Flush acquires a scope when called outside a frame, then discards residual encoder state before nanovg rendering (the shared encoder carries NativeEngine state).

Notes

• Tests.ExternalTexture.Msaa.cpp opens the next frame before waiting on startup() — a workaround for the legacy async AddToContextAsync pattern, obsoleted once Add synchronous CreateForJavaScript and deprecate AddToContextAsync in ExternalTexture #1646's sync CreateForJavaScript lands.
• Tests.ExternalTexture.RestoreAfterDeviceLoss (added on master after this branch) uses the same async pattern; the same frame-pump workaround is applied here. Obsoleted once Add synchronous CreateForJavaScript and deprecate AddToContextAsync in ExternalTexture #1646's sync CreateForJavaScript lands.
• bgfx: added m_encoderBeginLock in bgfx::frame() to prevent a deadlock with bgfx::destroy().
• Follow-up: removing the DeviceUpdate no-op shim is an API-breaking change handled in Remove DeviceUpdate usage and mark the shim deprecated #1653.

[Copilot]

Pull request overview

This PR reworks the graphics threading/synchronization model by replacing per-thread encoders and the SafeTimespan/UpdateToken mechanism with a single per-frame bgfx encoder owned by DeviceImpl and a new FrameCompletionScope gate/counter for cross-thread frame completion synchronization.

Changes:

• Removed SafeTimespanGuarantor, UpdateToken/Update, and per-thread encoder management; introduced a single frame encoder lifecycle in DeviceImpl.
• Added FrameCompletionScope + frame-start/before-render/after-render scheduling to coordinate JS-thread work with bgfx::frame().
• Updated NativeEngine/NativeXR/Canvas/FrameBuffer APIs to use GetActiveEncoder() and internal encoder acquisition patterns instead of passing encoders through call chains.

Reviewed changes

Copilot reviewed 21 out of 21 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
Core/Graphics/Source/DeviceImpl.h	Adds single-frame encoder pointer and frame gate (mutex/CV + pending scope count).
Core/Graphics/Source/DeviceImpl.cpp	Implements frame open/close sequencing, scope blocking, and frame-start dispatcher tick.
Core/Graphics/InternalInclude/Babylon/Graphics/DeviceContext.h	Replaces Update/UpdateToken with FrameCompletionScope, adds frame-start scheduler + encoder accessors.
Core/Graphics/Source/DeviceContext.cpp	Implements FrameCompletionScope RAII and forwards new scheduling/encoder methods.
Core/Graphics/Include/Shared/Babylon/Graphics/Device.h	Converts DeviceUpdate into a no-op compatibility shim and inlines GetUpdate.
Core/Graphics/Source/Device.cpp	Removes old GetUpdate implementation.
Core/Graphics/InternalInclude/Babylon/Graphics/FrameBuffer.h	Removes encoder parameters from bind/unbind/viewport/scissor APIs.
Core/Graphics/Source/FrameBuffer.cpp	Updates FrameBuffer internals to acquire view IDs without needing an encoder parameter.
Plugins/NativeEngine/Source/NativeEngine.h	Switches to active-encoder access + outside-frame scope holding; adds begin/end frame no-ops.
Plugins/NativeEngine/Source/NativeEngine.cpp	Updates command submission and RAF scheduling to use frame-start scheduling + scopes; updates ReadTexture blit scheduling.
Plugins/NativeEngine/Source/PerFrameValue.h	Adjusts API to remove encoder parameters (but currently has a type-signature issue).
Plugins/NativeXr/Source/NativeXrImpl.h	Removes stored Update member from XR session state.
Plugins/NativeXr/Source/NativeXrImpl.cpp	Migrates XR scheduling to frame-start + scopes; updates encoder usage.
Polyfills/Canvas/Source/Context.h	Removes stored Update member.
Polyfills/Canvas/Source/Context.cpp	Uses GetActiveEncoder() and acquires a FrameCompletionScope when flushing outside the frame cycle.
Polyfills/Canvas/Source/nanovg/nanovg_babylon.cpp	Updates FrameBuffer binding calls to new signature (no encoder param).
Apps/HeadlessScreenshotApp/Win32/App.cpp	Wraps startup wait with Start/Finish frame calls to keep gate open during startup.
Apps/StyleTransferApp/Win32/App.cpp	Same startup framing change as HeadlessScreenshotApp.
Core/Graphics/CMakeLists.txt	Removes SafeTimespanGuarantor sources from build.

Comments suppressed due to low confidence (1)

Plugins/NativeEngine/Source/PerFrameValue.h:33

• PerFrameValue is templated on T, but Set currently takes a bool parameter. This makes the template misleading and will break or silently convert if PerFrameValue is ever instantiated with a non-bool type. Change Set to accept T (e.g., const T& or T) to match the class template parameter.

        T Get() const
        {
            return m_value;
        }

        void Set(bool value)
        {
            m_value = value;
            if (!m_isResetScheduled)
            {

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[bghgary]

[Reviewed by Copilot on behalf of @bghgary]

LGTM.

[bkaradzic-microsoft]

🎉