This came up as part of the FSE Outreach Program's Rapid Revamp call for testing:

With the Details Block I felt very frustrated. In a first moment, I tried to drag and drop to move the selected content to “Type / to add a hidden block”, but it’s impossible why the Details block is closed when lost the focus. I had to use the keyboard (CMD + X) to cut the content and paste it. Additionally, I tried to click in the arrow to close the block (by intuition), but don’t occur anything. Only close when lost the focus and open when get the focus. In my programmer might, this block has an event onClick in the arrow (same that in the front), so perhaps it isn’t possible

I struggle for a while with block movement in general. I insert my block lower than I want, then selecting multiple paragraphs almost does me in. When I can’t keep details open to drag-and-drop, I resort to list view.

Here's a video showing how painful it is to drag and drop into:

In my mind, we need a way to visualize dropping into a nested container and/or have it open up, similar to what's been done with the List View and drag and drop #49563