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()