New submission from Terry J. Reedy <tjre...@udel.edu>:
When tk and hence, IDLE, opens a text window, it contains an integral number of
lines of text in the base font. Clicking a vertical scrollbar arrow button
moves text up or down exactly one line. Clicking in the trough between the
buttons and and the slider moves text up or down one 'page', a page being the
number of lines in the window -2.
The text and window can be de-synchronized with the slider and mouse wheel.
(If the window size is changes to a non-integral number of lines,
'synchronized' means a complete line at the top.) By default, the wheel moves
text about 3 lines per 'click'. Apparently, it is exactly 50 pixels at a time.
This is discussed on #25015. For that issue, the focus was on '3' (or 50)
being too small.
This issue is about changing 'about' to 'exactly', so that synchronized text
remains so when using the wheel or slider. For both, the solution should be to
use Text.xview_scroll(n, units).
For the wheel, we can replace the default tk wheel handler. While we are at
it, we could make the wheel effect be +- 5 lines, which would solve #25015,
which I will close. I don't think a configuration setting is needed.
For the slider, we can replace xview as the scrollbar command with a function
that maps command('moveto', fraction) to xview_scroll(n, 'units'). The number
n should be the difference between the current top line
("text.index("@0,0").split('.')[0]")
and the desired top line
("round(fraction * text.index('end').split('.')[0]").
Experiments should show if we need to adjust the rounded value
We should not need to adjust for the fact that the last <window-size> lines of
text cannot become top lines. A request to scroll 'too far' goes as far as
possible without raising an exception.
----------
assignee: terry.reedy
components: IDLE
messages: 317819
nosy: cheryl.sabella, terry.reedy
priority: normal
severity: normal
stage: test needed
status: open
title: IDLE: scroll text by lines, not pixels.
type: behavior
versions: Python 3.6, Python 3.7, Python 3.8
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Python tracker <rep...@bugs.python.org>
<https://bugs.python.org/issue33664>
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