Removing all subpanes from the Session pane prevents you from adding them back

Positron Version: 2024.05.0 (Universal) build 1294

Code - OSS Version: 1.89.0
Commit: 1ef95cdbc5ab3b79202339257802532b73e9d8c8
Date: 2024-05-23T04:27:50.357Z
Electron: 28.2.8
Chromium: 120.0.6099.291
Node.js: 18.18.2
V8: 12.0.267.19-electron.0
OS: Darwin arm64 23.4.0

Steps to reproduce the issue:

Don't do this unless you want to reset state :)

1. Drag all of the Variables, Viewer, and Plots panes out of the Session pane
2. Attempt to add them back or add new panes into the Session pane
3. You can only add them next to, not within the Session pane.

What did you expect to happen?

If we want to support the Session meta-pane, we should allow dropping elements within it. All other panes/elements allow stacking or rearranging, but the Session pane appears to take up the full vertical space and will no longer allow dropping in unless 1 or more panes are already there.

Were there any error messages in the output or Developer Tools console?

Reproduce the issue in the Session pane by dragging out the Variables, Viewer, and Plots panes, then try adding them back. Investigate the Session pane's drag-and-drop behavior; done means panes can be dropped within an empty Session pane as well as beside it.