Thank you for your support. The issue is now fixed in the latest update of InDesign.
Please upgrade to the new InDesign 14.0 from your Creative Cloud Application.
If you are unable to see the update yet, click on the menu (three dots) on the top right corner of Creative cloud app and click on “Check for App Updates”.
Let us know your feedback in the comment section.
InDesign TeamPhhomoMac commented
Forgot to write details: I use High Sierra on a non-retina iMac. I didn't observe any pattern (the problem occurs even just launching InDesign, without any open files). A simple relaunch of InDesign doesn't help.PhhomoMac commented
Is "Publish Online", in the preferences, deactivated? If yes, reactivate it, close InDesign, reopen it and tell me if panels expand. I did it and it worked.