Maybe Fission related. (DOMFissionEnabled=1)

Crash report: https://crash-stats.mozilla.org/report/index/10ef5f9e-49f8-4eca-870c-60dc90210822

MOZ_CRASH Reason: MOZ_RELEASE_ASSERT((mViewportSourceRect.x >= 0 && mViewportSourceRect.y >= 0 && mViewportSourceRect.width > 0 && mViewportSourceRect.height > 0) || (mViewportSourceRect.x == -1 && mViewportSourceRect.y == -1 && mViewportSourceRect.width == -1 && mViewportSourceRect.height == -1))

Top 10 frames of crashing thread:

0 libxul.so mozilla::layers::NativeLayerWayland::SetViewportSourceRect gfx/layers/NativeLayerWayland.cpp:686
1 libxul.so mozilla::layers::NativeLayerRootWayland::CommitToScreen gfx/layers/NativeLayerWayland.cpp:203
2 libxul.so mozilla::layers::NativeLayerRootWayland::CommitToScreen gfx/layers/NativeLayerWayland.cpp:139
3 libxul.so mozilla::wr::RenderCompositorNative::CompositorEndFrame gfx/webrender_bindings/RenderCompositorNative.cpp:254
4 libxul.so webrender::renderer::Renderer::render_impl gfx/wr/webrender/src/renderer/mod.rs:2124
5 libxul.so webrender::renderer::Renderer::render gfx/wr/webrender/src/renderer/mod.rs:1708
6 libxul.so wr_renderer_render gfx/webrender_bindings/src/bindings.rs:614
7 libxul.so mozilla::wr::RenderThread::UpdateAndRender gfx/webrender_bindings/RenderThread.cpp:491
8 libxul.so mozilla::wr::RenderThread::HandleFrameOneDoc gfx/webrender_bindings/RenderThread.cpp:347
9 libxul.so mozilla::detail::RunnableMethodImpl<mozilla::wr::RenderThread*, void  xpcom/threads/nsThreadUtils.h:1201

I recently set gfx.webrender.compositor.force-enabled to true.

Assert added by bug 1726807 comment 18.
Should be fixed by bug 1726807 comment 21.

Indeed should be fixed by the second patch in bug 1726807. But lets leave it open a bit longer to be sure.

Looks like there are still occasional cases where this crash can get triggered (build 20210824 certainly has the rounding fix), even though apparently much less likely. Now we need a reproducer for that cases.

This crash now has the signature of bug 1731450