tkinter.ttk.Scrollbar.fraction() under-estimates length

[pez]

BPO	33665
Nosy	@terryjreedy, @serhiy-storchaka, @E-Paine
Files	ttk_scroll_bug.py ttkscrl.py ttkscrl.tcl

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2020-07-03.05:31:48.428>
created_at = <Date 2018-05-28.10:39:05.799>
labels = ['3.8', 'type-bug', 'expert-tkinter', '3.9', '3.10']
title = 'tkinter.ttk.Scrollbar.fraction() under-estimates length'
updated_at = <Date 2020-07-03.05:31:48.427>
user = 'https://bugs.python.org/pez'

bugs.python.org fields:

activity = <Date 2020-07-03.05:31:48.427>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2020-07-03.05:31:48.428>
closer = 'serhiy.storchaka'
components = ['Tkinter']
creation = <Date 2018-05-28.10:39:05.799>
creator = 'pez'
dependencies = []
files = ['47617', '49292', '49293']
hgrepos = []
issue_num = 33665
keywords = []
message_count = 4.0
messages = ['317836', '318435', '372874', '372906']
nosy_count = 4.0
nosy_names = ['terry.reedy', 'serhiy.storchaka', 'pez', 'epaine']
pr_nums = []
priority = 'normal'
resolution = 'third party'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue33665'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']

[pez]

tkinter.Scrollbar(orient='horizontal').fraction() gives you an accurate representation of how far along the mouse is in the trough. The output is 0.0 when the mouse is on the first pixel of the trough. The output is 1.0 when the mouse is on the last pixel of the trough.

tkinter.ttk.Scrollbar(orient='horizontal').fraction() believes the trough is 16 pixels shorter than it is. The output is 0.0 when the mouse is on the first pixel of the trough. However, the output is greater than 1.0 when the mouse is on the last pixel of the trough. This extra 16 pixels is static, so the behavior is the same regardless of how wide the scrollbar is.

While possibly coincidental, it's interesting that the 16 pixels is also close to the default width of the scrollbar.

This is happening for me on Windows 7.
Did not test vertical orientation.

[terryjreedy]

Please post the code you used to test this.

[E-Paine]

I agree that the tkinter.ttk.Scrollbar.fraction under-estimates the length of the trough and I have written a program to try to measure this precisely.

When tested (only on Linux, granted), this deficit was measured to be 9 pixels (unless I got the maths wrong!). Tcl/Tk gave **exactly** the same numbers when run using 'wish'.

Thank you pez for reporting this, but 2 years on it should be closed as third party (please do take it up with the tcl team if you feel it is a real issue).

[serhiy-storchaka]

Thank you for testing it E. Paine. I agree that it is not a Tkinter issue.