To replicate the issue:

1. Open Debug mode
2. Switch to a view with tabs at the bottom (e.g. QML Profiler)
3. With your mouse, hover over the bottom of lower section header (where the "start debugging"/"enable profiling" etc. buttons are; see screenshot).

Result: The mouse cursor changes to a vertical resize one, but clicking and dragging does nothing. Dragging from the top of the header works fine.

This was found while testing the 13.0 beta, but can be replicated on 12.0.1.