gh-140389: fixed touchpad horizontal scrolling in IDLE editor

[TheLizzard]

Description

IDLE doesn't have a horizontal scrollbar but that shouldn't stop the user from using their trackpad/touchpad to horizontally scroll the text. This is a small bug that can sometimes annoy people

Previously

If you try to scroll horizontally using a trackpad/touchpad, the text in IDLE would scroll vertically.

Summary of changes

• Fixed IDLE's scrolling event handling (by respecting the direction embedded in the <tkinter.Event>.state)
• Added test cases to avoid regression in the future

Testing

I tested the issue and the fix on: Windows 11, Ubuntu 24.04 and Debian 13. The issue appears on all OSes and the fix works on all of them. While I am not 100% sure if this will break on MacOS, I am very confident that there will be no problems.

Fixes #140389

• Issue: IDLE: touchpad horizontal scrolling not working #140389

[TheLizzard]

Sorry for the typos I made in the tests I created and wasting GitHub resources. I was sure I ran them but I must have screwed up. Fixed them now and they should work.

[StanFromIreland]

I can confirm with manual testing that the patch does implement horizontal scrolling with a touchpad on Unix.

[github-actions]

This PR is stale because it has been open for 30 days with no activity.

[serhiy-storchaka]

I merged main into this branch to resolve the conflicts: gh-154511 (GH-156974) moved wheel_event() and its tests from idlelib.tree to idlelib.util, so the horizontal scrolling change is now applied there, with the tests in test_util.WheelEventTest. A second commit uses multicall.MC_SHIFT instead of the raw bit 1, and a third documents how Tk reports horizontal scrolling.

For the record, the Shift check is enough on every Tk we support: Tk 9 reports horizontal scrolling as <Shift-MouseWheel> on all systems, and since 8.6.10 X11 reports the horizontal wheel buttons 6/7 as <Shift-Button-4>/<Shift-Button-5> (Tk ticket 5da1d76e01); <Button-6> cannot even be bound in 8.6.

I have no strong opinion about this feature, but no objections either. I leave the decision to @terryjreedy.