If I stop mouse movement suddenly and release button, the page sometimes starts moving

[nyanpasu64]

Describe the bug
If I scroll a page, suddenly stop the mouse, and then release the mouse button while the mouse is nonmoving, the page sometimes starts moving.

Steps To Reproduce
Steps to reproduce the behavior:

1. Go to https://patorjk.com/misc/scrollingtext/timewaster.php
2. In scrollbar or smartphone mode, drag the page with the left or right button.
3. Stop the mouse abruptly.
4. Release the button.

Expected behavior
The page does not move.

Device info:

• operating system: Fedora 42
• Browser and version: Firefox 142.0.1 (64-bit)
• addon version: 9.2

Additional info
I think the problem is when you initiate momentum scroll when I release my mouse button, you take the velocity computed from the distance between the last 2-ish mouse cursor updates divided by the time difference. This breaks when the mouse doesn't move at all before releasing it, but it was last moving at high speed.

If the mouse is released after the last mouse movement update, you should instead use the distance between the last mouse movement and the position of the release to compute momentum scrolling.

Question: How long between a position update and a button release, before we treat it as nonmoving?

• At all?
• More than 10ms? 100ms?
• More than 2x the polling rate?
  • But you don't know the polling rate, and just because the user moves the mouse every 500ms you shouldn't treat a mouse release as a momentum scroll because it happens 400ms after moving the mouse.

[Juraj-Masiar]

Oh, these are some tough questions :D.
It's been 6 years since I've implemented the Momentum feature, which is definitely responsible for the jumps.

From what I can see in the code, I'm not polling in time intervals, instead I'm taking some of the last values used for scrolling:

  // computes time deltas AND then remove high and low values (something like a median)
  // from up-to 10 input values we get 9 deltas and we return 5 middle values. Those are later used to compute average.
  private computeDeltasAndNormalize(array: [number, number][]): [number, number][] {
    const result: [number, number][] = [];
    for (let i = 1; i < array.length; ++i) {
      // s - distance; t - time; ps - previous distance
      const [s, t] = array[i], [ps, pt] = array[i - 1];
      const ds = s - ps;
      const dt = t - pt;
      result.push([ds, dt]);
    }
    console.log('deltas', result);
    // Removing 2 highest and 2 lowest values:
    // Why? First - Chrome tent to "jump" from 2000 to 0 for no reason. Second, it seems to yield better results. Something like a median.
    for (let i = 0; i < 2 && result.length > 3; i++) {
      const values = result.map(([d]) => d);
      const min = Math.min(...values);
      const max = Math.max(...values);
      removeOneFromArray(result, ([d]) => d === min);
      removeOneFromArray(result, ([d]) => d === max);
    }
    console.log('deltas', result);
    return result;
  }

I can't remember reasons for this though, but it doesn't sound very good, now that I think about it...
That being said, I remember the situation is much more complex than it sounds.

There were also browser bugs as mentioned in the comment and then there was a "real world issue" - releasing the mouse button while stopping the mouse movement is tricky. It's not like on the phone where the finger naturally goes up. Here as you release the mouse button, whole mouse can "jump" a bit.

But yeah, current solution is likely affected by the monitor refresh rate, or maybe mouse polling rate, I have a feeling OS will send the mouse move events even when screen doesn't change.

Actually, checking my git history now, I was using time based polling, with a 1ms resolution and it I've changed it for some reason:

So anyway, I should probably make it more configurable, or maybe just try to fix it. Now that I'm a more skilled programmer, maybe I can think of something better. Then again, if there is not enough "values" to work with, it may be impossible to tell whether user wanted to activate the momentum or stop scrolling.