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UCEERR_ILLEGALPACKET
UCEERR_ILLEGALPACKET is a Windows User Composition Engine (UCE) HRESULT. It indicates that the packet type is known but cannot be processed at the current stage of the composition-channel lifecycle.
What to check
- Check the order of UI, layout, rendering, and teardown operations around the failure.
- Avoid updating or using a visual after its dispatcher, window, or composition target begins shutting down.
- Reduce concurrent UI updates to confirm whether an invalid lifecycle transition is involved.
Microsoft guidance for WPF render-thread failures
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