UV version:

 universalviewer@4.2.0RC2

I'm submitting a:

• bug report => please fork one of these codesandbox examples with a repro of your issue and include a link to it below
• feature request => please use the user stories repo
• support request => Please do not submit support requests here, use stackoverflow

Current behavior:
If the viewer is in a state with an open left panel and a closed right panel, loading a multi-part PDF manifest causes a blank left panel to appear, like this:

Expanding the right panel causes the left panel contents to also appear, like this:

After this point, everything behaves normally.

Expected behavior:
The left panel should never be unexpectedly empty.

Steps to reproduce:

Go to https://universalviewer-git-fork-jamesmisson-release-420-rc2-mnemoscene.vercel.app/#?xywh=-2593%2C-382%2C7754%2C4305&iiifManifestId=https%3A%2F%2Fdigital.library.villanova.edu%2FItem%2Fvudl%3A294631%2FManifest -- if the problem does not immediately appear, make sure that the left panel is open and the right panel is closed, then refresh the browser. You should see the problem.

Related code:

I strongly suspect this is related to recent changes to panel behavior introduced for rc2.