It looks like you're a bit confused about the different editing modes in Joplin.
Joplin has 2 editors, the markdown editor and the WYSIWYG editor.
With the markdown editor you can toggle between the editor alone, the viewer alone, and split view (Ctrl-L, Cmd-L).
The WYSIWYG editor does not have separate modes.

AFAIK there isn't a keybinding to switch between the markdown and the WYSIWYG editors.

So if you do want quick switch of viewing/editing and you don't like the split view. Just select "Editor/Viewer" under View -> Layout Button Sequence.
If you do indeed want to toggle between the WYSIWYG editor and the markdown editor, you are unfortunately out of luck for now.

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