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Another reason to replace IDLE's custom tree widget with hard-coded constants with ttk.Treeview.
I am closing this because I believe that #31552 will solve this problem. For an initial check, run the following test code, extracted from
https://tkdocs.com/tutorial/tree.html
Click the [+] buttons. I am curious whether commenting out the fix_scaling call makes any difference.
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import tkinter as tk
from tkinter import ttk
from idlelib.run import fix_scaling
root = tk.Tk()
tree = ttk.Treeview(root)
tree.pack()
fix_scaling(root)
tree.insert('', 'end', 'widgets', text='Widget Tour')
tree.insert('', 0, 'gallery', text='Applications')
id = tree.insert('', 'end', text='Tutorial')
tree.insert('widgets', 'end', text='Canvas')
tree.insert(id, 'end', text='Tree')
root.mainloop()