[JoshCLWren]

Problem

Mouse-wheel scroll up did nothing and every notch scrolled/advanced down, in the packaged builds. Logs from a real run show it:

Mouse wheel delta=120  num=??  ->  Scroll by delta=216   # up, but offset increases
Mouse wheel delta=-120 num=??  ->  Scroll by delta=216

Tk 9 delivers <MouseWheel> on X11 with a +/-120 delta and reports the unused button substitution as the string "??". The old handler was:

if hasattr(event, "num") and event.num:      # "??" is truthy
    direction = -1 if event.num == 4 else 1  # "??" == 4 is False -> always 1 (down)

So every notch decoded as scroll-down; scroll-up was unreachable. Tk 8.6 delivered <Button-4>/<Button-5> with a real int num, which is why source builds worked and packaged (Tk 9) builds did not.

Fix

• _wheel_direction() reads delta first and only treats num as a button when it is actually 4 or 5.
• _wheel_step() uses a finer step (1/10 of the canvas height, min 40px) so the wheel is smoother than the arrow-key step.

Testing

• Regression tests cover the real Tk 9 num="??" for scroll-up, scroll-down, prev-page navigation, plus the ignored-event case.
• bash scripts/lint.sh clean; xvfb-run pytest: 562 passed, coverage 96.38%.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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