undoStudio has one editor panel. A Structured Text file, a JSON file, a C++ file and a plain text file all open in it, each in its own tab, above the same tab bar.

the Editor panel with three files open, one marked with an asterisk

a .st file open, Variables above Body, with the splitter between them

The tab bar


A click keeps the tab	There is one preview slot, and browsing the tree through it replaces what was there. A file somebody asked to see is still there when they ask for another one.
A second click is not needed	ImGui only reports a double click when both clicks land within a few pixels of each other — a rule nobody can see.
An asterisk means unsaved changes	It is the file's own text that is asked, not a flag set when the tab was left. Browsing past a file with edits pins it rather than throwing the typing away.
`Ctrl+S` saves	The button is there too, in the ST toolbar.
Closing a tab unloads the file	Closing the project clears them all.
Renaming follows the file	A rename or a move in the tree rewrites the tab, the path a save goes to, and the key its text is held under — all four, or `Ctrl+S` writes a copy where the original used to be. A deleted file takes its tab with it.

A file that is not on screen is not kept in memory as text on disk: its unsaved contents are stashed, and the stash is handed back when the tab is selected again.

Which backend a file gets

Extension	Backend	Notes
`.st`	Structured Text	structured-text.md
`.json`	JSON, or Text if it is project configuration	json.md
`.c` `.cpp` `.cc` `.cxx` `.h` `.hpp` `.hh` `.hxx`	C++	Syntax colouring from ImGuiColorTextEdit's own definition
anything else	Text	No colouring

Which backend a .json gets is the file's role, not its extension, answered from the project that is open. That is why a .json outside any project is a tree and the same extension inside one is text: the configuration is the file the user is told to edit by hand, and a tree has nothing to type into.

Switching a <tt>.json</tt> between text and tree

Right-click any .json in the Workspace tree:

Open as Text — the editable view
Open as Tree — the browsable view

Both replace the tab rather than adding a second one. One file is one row in the bar, and the row is whichever view it is in.

Panels beside it

Panel	What is in it
Workspace	The file tree, and the project toolbar — workspace.md
ST Output	Build and validation messages — output.md
ST Outline	The POU, its sections, methods, structs and enums, from the parsed file — structured-text.md
undoApp Terminal	An embedded shell — terminal.md

All four are listed under View and can be shown or hidden, and View > Reset Layout puts them back where the shipped layout has them.