Description

johnohagan

opened on Mar 10, 2025

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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

Activity

1. johnohagan

   added

   type-bugAn unexpected behavior, bug, or errorAn unexpected behavior, bug, or error

   on Mar 10, 2025
2. picnixz

   added

   stdlibStandard Library Python modules in the Lib/ directoryStandard Library Python modules in the Lib/ directory

   topic-tkinter

   on Mar 10, 2025
3. johnohagan commented on Apr 6, 2025

   

   johnohagan

   on Apr 6, 2025

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   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?
4. terryjreedy

   added

   OS-linux

   on Apr 6, 2025
5. terryjreedy commented on Apr 6, 2025

   

   terryjreedy

   on Apr 6, 2025

   Member

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   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.
6. johnohagan commented on Apr 7, 2025

   

   johnohagan

   on Apr 7, 2025

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   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')
7. ThingamabobsCoding commented on Feb 21, 2026

   

   ThingamabobsCoding

   on Feb 21, 2026

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   This looks a lot like the window manager posting weird events to tkinter.
   Watching your video shows that when you stop resizing the window a "<B1-Motion>" event is performed as described in the manpage in section bindings:

   If the mouse is dragged out of the widget while button 1 is pressed, the entry will automatically scroll to make more text visible (if there is more text off-screen on the side where the mouse left the window).
   https://www.tcl-lang.org/man/tcl/TkCmd/text.html#M192

   Weirdly enough, you show how you resize the window to increase the width afterwards and no event gets posted, which also should stop this.
8. ThingamabobsCoding commented on Mar 2, 2026

   

   ThingamabobsCoding

   on Mar 2, 2026

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   Note that there is no wayland support for tk

   https://core.tcl-lang.org/tk/tktview/3f58c0871c

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