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## Summary of the Pull Request
Add functionality to move a pane to another tab. If the tab index is greater than the number of current tabs a new tab will be created with the pane as its root. Similarly, if the last pane on a tab is moved to another tab, the original tab will be closed.
This is largely complete, but I know that I'm messing around with things that I am unfamiliar with, and would like to avoid footguns where possible.
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## References
#4587
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## PR Checklist
* [x] Closes#7075
* [x] CLA signed. If not, go over [here](https://cla.opensource.microsoft.com/microsoft/Terminal) and sign the CLA
* [ ] Tests added/passed
* [x] Documentation updated. If checked, please file a pull request on [our docs repo](https://github.com/MicrosoftDocs/terminal) and link it here: #xxx
* [x] Schema updated.
* [ ] I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #xxx
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## Detailed Description of the Pull Request / Additional comments
Things done:
- Moving a pane to a new tab appears to work. Moving a pane to an existing tab mostly works. Moving a pane back to its original tab appears to work.
- Set up {Attach,Detach}Pane methods to add or remove a pane from a pane. Detach is slightly different than Close in that we want to persist the tree structure and terminal controls.
- Add `Detached` event on a pane that can be subscribed to to remove other event handlers if desired.
- Added simple WalkTree abstraction for one-off recursion use cases that calls a provided function on each pane in order (and optionally terminates early).
- Fixed an in-prod bug with closing panes. Specifically, if you have a tree (1; 2 3) and close the 1 pane, then 3 will lose its borders because of these lines clearing the border on both children https://github.com/microsoft/terminal/blob/main/src/cascadia/TerminalApp/Pane.cpp#L1197-L1201 .
To do:
- Right now I have `TerminalTab` as a friend class of `Pane` so I can access some extra properties in my `WalkTree` callbacks, but there is probably a better choice for the abstraction boundary.
Next Steps:
- In a future PR Drag & Drop handlers could be added that utilize the Attach/Detach infrastructure to provide a better UI.
- Similarly once this is working, it should be possible to convert an entire tab into a pane on an existing tab (Tab::DetachRoot on original tab followed by Tab::AttachPane on the target tab).
- Its been 10 years, I just really want to use concepts already.
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## Validation Steps Performed
Manual testing by creating pane(s), and moving them between tabs and creating new tabs and destroying tabs by moving the last remaining pane.
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