HdMergingSceneIndex re-adds removed prims during UsdImaging stage change

[lanthia-adsk]

Hi,

We are encountering an issue in MayaHydra where prims from a previous USD source are not removed when the source file of a USD node is changed, resulting in overlapping geometry from the old and new stages in the viewport.

We get the following call sequence :

1. void UsdImagingStageSceneIndex::SetStage(UsdStageRefPtr stage) was called with a null stage.
2. UsdImagingStageSceneIndex::_SendPrimsRemoved({SdfPath::AbsoluteRootPath()}); was called to remove all prims.
3. void HdMergingSceneIndex::_PrimsRemoved(const HdSceneIndexBase &sender, const HdSceneIndexObserver::RemovedPrimEntries &entries) was called and forwarded the removal by

   OpenUSD/pxr/imaging/hd/mergingSceneIndex.cpp

   Line 560 in d8db3b4

   _SendPrimsRemoved(entries);

    However, after the removal propagation,

   OpenUSD/pxr/imaging/hd/mergingSceneIndex.cpp

   Line 561 in d8db3b4

   _SendPrimsAdded(addedEntries);

   is called, which re-adds all previously removed prims.

This causes the prims from the old USD source remain active alongside prims from the new source, causing overlapping objects in the Hydra viewport. The stale prims are only cleared when the Hydra viewport is fully destroyed and recreated (for example, by switching viewports).

By reverting commit b4112c7, the problem goes away. We believe this might be similar to issue #3563 .

Thank you.

[jesschimein]

Filed as internal issue #USD-11852

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