Changing terrain provider moves billboard to incorrect location
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Description
This [came up on the forum](https://groups.google.com/d/msg/cesium-dev/WOPt3-BX_9s/kZMVqYENDAAJ). Here's a [Sandcastle example](https://cesiumjs.org/Cesium/Build/Apps/Sandcastle/#c=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). To reproduce:
1. Zoom in close enough to the billboards and tilt the camera until you see them curve

2. Select "delete billboards"
3. Select "change terrain provider"
4. Select "add billboards"
5. Select "change terrain provider"
6. Observe that they're now in a different position:

This may be just another manifestation of https://github.com/AnalyticalGraphicsInc/cesium/issues/4776 but I couldn't dig deep enough to figure out what the underlying issue was.
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Start with the linked Sandcastle example and follow the six reproduction steps, especially the billboard and terrain-provider changes. Then trace the billboard and terrain-provider update paths to determine where their positions are recalculated. Done means repeated terrain-provider changes leave the billboards in the same correct locations.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- javascript
- Domain
- computer-graphics
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Mostly clear
- Newbie friendliness
- 35/100