Qt wheelEvent: use x coordinate if ALT is press

[GerardDorado]

PR summary

Based on what I found here and here it looks like Qt default behaviour is to reverse the axis coordinates for the WheelEvent when the 'alt' key is pressed, so I made a small modification to the wheelEvent method inside the FigureCanvasQT class to check if the key is pressed, if so, instead of using angleDelta().y() we use angleDelta().x() to get the correct vertical movement.

I couldn't find a reference to the x axis anywhere in the code, and since the wheelEvent method only used the vertical movement, i don't think this would rise an issue in the future.

I don't know if this happens for pixelDelta() , since I can't test it.

PR checklist

• "closes [Bug]: Mouse Scroll Event is not caputred if ALT key is pressed #25671 " is in the body of the PR description to link the related issue
• new and changed code is tested
• [N/A] Plotting related features are demonstrated in an example
• [N/A] New Features and API Changes are noted with a directive and release note
• [N/A] Documentation complies with general and docstring guidelines

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[oscargus]

Thanks! I think that this makes sense, but will leave it to others to determine if it is a feature that we want. Will also chip in that it is more or less impossible to test wheelEvent in Qt (and we do not even use pytest-qt).

[tacaswell]

Thank you for working on this, however I am currently 👎 on taking this change.

That we do not currently deal with horizontal scroll is arguably a bug (or at least a missing feature) so I have concerns that this would pin us in the future.

My other concern is a bit philosophical. I think of our event system as being "the last in line" of a long chain of software between the user pressing a physical key and the mpl-side callback being notified and as such we should trust the layers between us and the users to have done their jobs correctly / as intended and we should no second-guess those layers. For example, there are some keys the user hits that get intercepted by the window manager (for example "change my virtual desktop") that never even make it to the GUI application we are wrapped in. In general we have no visibility into what (or why) those layers are doing and while it may seem "obvious" in some cases, what we should (un)do, it is impossible to be sure we got it right so we should not do anything. If the issue is behavior at a level closer to the user, the user should fix that layer, rather than sometimes maybe fixing it for some users and leaving users who did want that behavior stuck.

[tacaswell]

See my long comment.

TL;DR

• we may want to support horizontal scroll events in the future
• we should not second-guess Qt

If consensus goes the other way anyone can dismiss this review.

[tacaswell]

Thank you for your work on this @GerardDorado ! Even if we do not take this patch, your research on this being expected Qt behavior is extremely valuable.

[anntzer]

I agree with @tacaswell's assessment.
Feel free to ping for reopen if you disagree.

[GerardDorado]

Thanks everyone for the feedback!, It was my first time contributing to open source so I'm glad I could help a little

[tacaswell]

@GerardDorado Please do not be discouraged and I hope we hear from you again.
Everyone who works on open source regularly has some declined PRs, it is part of the process.

Even though we did not merge your PR, your work did result in closing #25671 which is a contribution to Matplotlib (even if it won't show up in the git history or GH's green-square metrics (which one of the reasons those metrics should be treated with a bit of skepticism but I digress))!