FSceneViewport::UpdateViewportRHI() Assertion failed: IsInGameThread()

Development Programming & Scripting
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Hello, since integrating 5.8.0-preview-1 we’ve started failing an IsInGameThread() check in FSceneViewport::UpdateViewportRHI() sporadically. It happens during teardown for different maps while running test automation in editor. The log immediately prior to the assert looks like this:

[2026.06.10-15.50.55:134][634]LogWorld: BeginTearingDown for /HavokNavigationTest/Maps/Test/UEDPIE_0_NavMeshFindPathTest

[2026.06.10-15.50.55:146][634]LogWorld: UWorld::CleanupWorld for NavMeshFindPathTest, bSessionEnded=true, bCleanupResources=true

[2026.06.10-15.50.55:158][634]LogSlate: InvalidateAllWidgets triggered. All widgets were invalidated

[2026.06.10-15.50.55:170][634]LogHavokNavTest: SamsaraWorldSubsystem_0 (000001F5391CB900): USamsaraWorldSubsystem::Deinitialize ()

[2026.06.10-15.50.55:182][634]LogPlayLevel: Display: Shutting down PIE online subsystems

[2026.06.10-15.50.55:194][634]LogSlate: InvalidateAllWidgets triggered. All widgets were invalidated

[2026.06.10-15.50.55:206][634]LogAudio: Display: Audio Device unregistered from world ‘NavMeshFindPathTest’.

[2026.06.10-15.50.55:218][634]LogAudioMixer: Deinitializing Audio Bus Subsystem for audio device with ID 100

[2026.06.10-15.50.55:230][634]LogAudioMixer: Display: FAudioMixerWasapi::StopAudioStream() InstanceID=100, StreamState=4

[2026.06.10-15.50.55:242][634]LogAudioMixer: Display: StopRunningNullDevice() called, InstanceID=100

[2026.06.10-15.50.55:254][634]LogSlate: Updating window title bar state: overlay mode, drag disabled, window buttons hidden, title bar hidden

That’s all the info I have, hope it’s helpful, thank you!

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3

I’m not sure if the call stack was captured since I don’t see it here, so pasting it here just to be sure:

* Fatal Error: Assertion failed: IsInGameThread() [File:D:\GH\18f2bb4c1110\Engine\Source\Runtime\Engine\Private\Slate\SceneViewport.cpp] [Line: 2104]

0x00007ffe324eed88 UnrealEditor-Core.dll!FDebug::CheckVerifyFailedImpl2() [D:\GH\18f2bb4c1110\Engine\Source\Runtime\Core\Private\Misc\AssertionMacros.cpp:797]

0x00007ffe2d8df0f7 UnrealEditor-Engine.dll!FSceneViewport::UpdateViewportRHI() [D:\GH\18f2bb4c1110\Engine\Source\Runtime\Engine\Private\Slate\SceneViewport.cpp:2104]

0x00007ffe2d86c1cd UnrealEditor-Engine.dll!FSceneViewport::~FSceneViewport() [D:\GH\18f2bb4c1110\Engine\Source\Runtime\Engine\Private\Slate\SceneViewport.cpp:111]

0x00007ffe2d8748a3 UnrealEditor-Engine.dll!FSceneViewport::`vector deleting destructor’() [Unknown File]

0x00007ffe1104fc78 UnrealEditor-SlateRHIRenderer.dll!SharedPointerInternals::TReferenceControllerBase<1>::ReleaseSharedReferenceNoInline() [D:\GH\18f2bb4c1110\Engine\Source\Runtime\Core\Public\Templates\SharedPointerInternals.h:259]

0x00007ffe11022581 UnrealEditor-SlateRHIRenderer.dll!FSlateRHIRenderer::DrawWindow_RenderThread() [D:\GH\18f2bb4c1110\Engine\Source\Runtime\SlateRHIRenderer\Private\SlateRHIRenderer.cpp:1237]

0x00007ffe11024421 UnrealEditor-SlateRHIRenderer.dll!FSlateRHIRenderer::DrawWindows_RenderThread() [D:\GH\18f2bb4c1110\Engine\Source\Runtime\SlateRHIRenderer\Private\SlateRHIRenderer.cpp:1475]

0x00007ffe11005238 UnrealEditor-SlateRHIRenderer.dll!`FSlateRHIRenderer::DrawWindows_Private’::`76’::<lambda_3>::operator()() [D:\GH\18f2bb4c1110\Engine\Source\Runtime\SlateRHIRenderer\Private\SlateRHIRenderer.cpp:1738]

0x00007ffe2b676373 UnrealEditor-RenderCore.dll!ExecuteCommand() [D:\GH\18f2bb4c1110\Engine\Source\Runtime\RenderCore\Private\RenderingThread.cpp:1531]

0x00007ffe2b6499bc UnrealEditor-RenderCore.dll!UE::RenderCommandPipe::FCommandList::ConsumeCommands<`FRenderThreadCommandPipe::ExecuteCommands’::`2’::<lambda_1> >() [D:\GH\18f2bb4c1110\Engine\Source\Runtime\RenderCore\Public\RenderingThread.h:335]

0x00007ffe2b6664f1 UnrealEditor-RenderCore.dll!`FRenderThreadCommandPipe::EnqueueAndLaunch’::`5’::<lambda_1>::operator()() [D:\GH\18f2bb4c1110\Engine\Source\Runtime\RenderCore\Private\RenderingThread.cpp:1926]

0x00007ffe2b676bec UnrealEditor-RenderCore.dll!TGraphTask<TFunctionGraphTaskImpl<void __cdecl(void),1> >::ExecuteTask() [D:\GH\18f2bb4c1110\Engine\Source\Runtime\Core\Public\Async\TaskGraphInterfaces.h:703]

0x00007ffe32172f43 UnrealEditor-Core.dll!UE::Tasks::Private::FTaskBase::TryExecuteTask() [D:\GH\18f2bb4c1110\Engine\Source\Runtime\Core\Public\Tasks\TaskPrivate.h:524]

0x00007ffe32166c2d UnrealEditor-Core.dll!FNamedTaskThread::ProcessTasksNamedThread() [D:\GH\18f2bb4c1110\Engine\Source\Runtime\Core\Private\Async\TaskGraph.cpp:807]

0x00007ffe32166f9e UnrealEditor-Core.dll!FNamedTaskThread::ProcessTasksUntilQuit() [D:\GH\18f2bb4c1110\Engine\Source\Runtime\Core\Private\Async\TaskGraph.cpp:696]

0x00007ffe2b6961b3 UnrealEditor-RenderCore.dll!RenderingThreadMain() [D:\GH\18f2bb4c1110\Engine\Source\Runtime\RenderCore\Private\RenderingThread.cpp:261]

0x00007ffe2b6968b4 UnrealEditor-RenderCore.dll!FRenderingThread::Run() [D:\GH\18f2bb4c1110\Engine\Source\Runtime\RenderCore\Private\RenderingThread.cpp:390]

0x00007ffe3288b928 UnrealEditor-Core.dll!FRunnableThreadWin::Run() [D:\GH\18f2bb4c1110\Engine\Source\Runtime\Core\Private\Windows\WindowsRunnableThread.cpp:167]

0x00007ffe3288548a UnrealEditor-Core.dll!FRunnableThreadWin::GuardedRun() [D:\GH\18f2bb4c1110\Engine\Source\Runtime\Core\Private\Windows\WindowsRunnableThread.cpp:87]

0x00007ffe7e154cb0 KERNEL32.DLL!UnknownFunction [Unknown File]

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Hello,

Thanks for reporting this, it’s a known unresolved crash we’re aware of but haven’t gotten to yet. It’s likely due to a race condition between the game/render threads stemming from how SlateRHIRenderer.cpp creates a temporary TSharedPtr in FSlateRHIRenderer::DrawWindow_RenderThread

DrawWindowViewport_RenderThread(
    Inputs.Window->GetViewport().Get()

If you’re able to reproduce this error you can try working around it by adding ISlateViewport* to FWindowToRender and FSlateDrawWindowPassInputs and set it in DrawWindows_Private near the code that creates the FWindowToRender

ISlateViewport* Viewport = Window->GetViewport().Get();
if (Viewport)
{
    // ... existing code using Viewport ...
}
// Later:
WindowsToRender.Emplace(FWindowToRender { ..., .SlateViewport = Viewport });

Then, replace the temp TSharedPointer with

DrawWindowViewport_RenderThread(
      Inputs.SlateViewport

If you’re able to share a small repro with us that would also be very helpful, especially if it’s in vanilla UE.

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