*This is a pre-existing issue that I noticed while working on PR 12613, and fixing this also brings the thumbnail code inline with the page code.*
Given the intermittent nature of all of this, it's somewhat difficult to reproduce it consistently; however the following steps should at least provide an outline:
1. Open the sidebar, and the thumbnailView, and start scrolling around.
2. *Quickly* close the sidebar, so that all thumbnails won't have time to finish rendering.
3. Either wait for the cleanup-timeout to occur, or simply run `PDFViewerApplication.cleanup()` in the console.
What *intermittently* happens here is that `WorkerTransport.startCleanup` rejects, and consequently that cleanup doesn't complete as intended, since some of the thumbnails are left in a *pending* renderingState[1].
Fixing this is simple though, and only requires updating `PDFThumbnailViewer.cleanup` along the lines of `BaseViewer.cleanup`.
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[1] Keep in mind that thumbnails will *only* render when the thumbnailView is visible, to reduce resource usage.
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