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A necessary prerequisite of tabbed windows, editor and shell in same window etc. as per #24826, is that editors stop thinking they are in their own toplevel window.
The attached component.patch is unfortunately long, but this was necessary due to the highly inter-related nature of the changes. It results in no functionality changes to IDLE at this point, just a reorganization.
In summary, WindowList.ListedToplevel was abstracted into a more general Container, which will later be expanded to do more than just a simple toplevel (though that is all it is now). EditorWindow (and friends) no longer call window operations like wm_title themselves, but go through a Container API.
At the same time, a new Component base class was created, which EditorWindow and several others now inherit from. Components get inserted into Containers of course, and the base class ensures certain things the Container needs are present. (The Component also can respond to callbacks from other Components sent via the file list; this approach is used in the patch in #25031 for configuration changes).
Finally, what remained of WindowList (effectively a simplified WindowList class, but not ListedTopLevel) was folded into FileList.
In terms of understanding and/or reviewing this patch, I'd recommend the following:
 1. start with container.py
 2. then component.py
 3. then a simple example of how it's applied, ClassBrowser.py
 4. a slightly more verbose, but equally simple example, Debugger.py
 5. FileList.py (mostly comes from WindowList, but note how used by container.py
 6. then EditorWindow.py

FYI, I've gone past this point in my own playground, to the point of having a working tabbed editor implementation. It's not that much further actually. Here are the next steps:
1. Change so that Container is passed to Components on creation, rather than Component creating its own container.
2. Change statusbar so that it's in control of what is displayed, and EditorWindow just pings it to update itself; statusbar calls into EditorWindow and chooses what info to get.
3. Change so that container decides how to display title, EditorWindow just says title has changed. Add saved/dirty status info to Container.
4. Create Container subclass TabbedContainer using the separate uitabs.py widget I put together. Manages the tabs and switching in different content as tabs are switched. Altogether < 100 lines of code.
5. Create Container subclass ProxyContainer, representing what's managed by a single tab. Just passes things off to TabbedContainer. < 50 lines of code.
6. Change FileList to created a TabbedContainer and then ProxyContainers for each shell/editor added.