Tkinter text widget scrolling uncontrollably under Wayland

[johnohagan]

Bug report

Bug description:

On Gnome with Wayland (current Debian testing), resizing a Tk root window to increase the height causes a contained Text widget to scroll uncontrollably to the bottom of the widget. If the widget is scrolled while this is happening, it keeps scrolling back to the bottom.

Resizing the window again by decreasing the height or changing the width, or clicking inside the text widget, usually stops the issue.

This happens with both the mouse scrollwheel and the tkinter scrollbar.

This doesn't happen on Gnome using X, or on Windows 10. I haven't tested on any other systems.

Minimal example:

from tkinter import *
from random import choice
from string import ascii_letters

root = Tk()
textwidget = Text(root)

for i in range(1000):
    textwidget.insert(END, f'{choice(ascii_letters)} ')

textwidget.pack(fill=BOTH, expand=True)
mainloop()

A screen-recording is attached using the above code and showing the issue, which occurs the third time the window is resized.

Screencast.From.2025-03-10.13-20-57.mp4

I have no idea how to debug this in more depth, any suggestions welcomed.

CPython versions tested on:

3.13

Operating systems tested on:

Linux, Windows

[johnohagan]

I found a workaround: the uncontrolled scrolling is prevented by binding the '' event of the text widget to a function that returns the string 'break', which AIUI prevents default event propagation. Hopefully this is a clue about why it is happening?

[terryjreedy]

A more minimal example would be to replace the imports and loop with something like textwidget.insert(END, 'a b c d e f '*150). Either way, the text widget ends up with 1 long line, which is wrapped by default. On Windows, I don't see any auto scrolling. Perhaps Wayland prefers showing last physical line of a single wrapped line.

I do not understand '' event. What is the actual bind call?

I strongly suspect that this is a bug or anomaly in the interaction of Wayland and tk, and that tkinter has nothing to do with the bahavior. Once mainloop is called, tkinter is not involved. So this should likely be closed.

[johnohagan]

I used a random letter sequence so that the effect of the wrapping is easier to see.

Note that the unwanted scrolling doesn't occur on Windows for me either.

Regarding the event, I wrote '<Leave>' but didn't realise I needed to escape the '<>' characters. My apologies. The additional code was:

textwidget.bind('<Leave>', lambda x: 'break')