Looks like you've pulled out the panels (undocked). You can restore the original layout using File->Restore Layout.
I would also suggest you to lock the layout to prevent docking/undocking by File->Lock Layout->Prevent docking. This will save yourself from mistakingly undocking panels.
[MacBook Pro (desktop only); PostgreSQL12; PGAdmin 4.15]
Can someone please tell me where the setting is for making the panel borders visible? I had great borders in PgAdmin 4.15 two days ago. A re-install of 4.15 and a search of the manual (Query Tool preferences) failed to help.