Splitting primary/secondary sidebars by dragging the tool's icon is impossible in certain situations

[Nowaker]

Does this issue occur when all extensions are disabled?: Yes - When disabled, the issue is reproducible by trying to drag Extensions tool into Outline tool.

• VS Code Version: 1.101.0
• OS Version: Arch Linux 6.15.2-arch1-1

Starting state of the system:

1. Ask Copilot is a splittable tool. (For whatever reason)
2. Roo, Cline, and most other tools are non-splittable tools. (...Until they become splittable)

Steps to Reproduce:

1. Scenario 1: Drag anything into splittable Ask Copilot to split the sidebar. -> Sidebar splits. Revert.
2. Scenario 2: Drag anything into non-splittable Roo to split the sidebar. -> It won't split.
3. Scenario 3:
   1. Drag anything non-splittable (here: Roo) into Ask Copilot to split the sidebar. -> Sidebar splits.
   2. Drag anything non-splittable (here: Cline) into Ask Copilot + Roo. -> Sidebar splits even further.
   3. Move Ask Copilot out. The sidebar now consists of two previously non-splittable tools.
   4. Drag anything (previously) non-splittable Roo + Cline sidebar. -> Sidebar splits even further. Revert.
      Conclusion: Through the powers of a splittable tool Ask Copilot, we were able to split two originally non-splittable tools.
4. Scenario 4: Repeat Scenario 2. -> It won't split.
   Conclusion: Roo and Cline are non-splittable tools again. They can only be split again through the powers of Ask Copilot.

This is best visualized with a video:

vscode-sidebar-splitting-2025-06-19_12.52.46.mp4

[vs-code-engineering]

Thanks for creating this issue! It looks like you may be using an old version of VS Code, the latest stable release is 1.101.1. Please try upgrading to the latest version and checking whether this issue remains.

Happy Coding!