Clamp scroll position within bounds in the ui/scroll.rs example

[JDDev0]

Objective

• The scroll position is not bound in the on_scroll_handler in the ui/scroll.rs example. (This is only a problem in combination with scrollbars. If scrollbars are present they could slightly overshoot.)

Solution

• This PR clamps the scroll position between 0.0 and the max_position value.

Testing

• I tested those changes by running the Scroll example on Ubuntu (Wayland). Scrolling still works. I tested those changes in my own bevy project in combination with scrollbars and this change fixed the problem.

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[alice-i-cecile]

Sensible fix, but a comment on the math would probably be useful for readers.

[ickshonpe]

Very sleepy but isn't the scroll position clamped during layout anyway, why is this necessary?

There is something wrong though somewhere because I just tried adding scrollbars to the example and they undershoot now with or without this change. I'll take another look tomorrow.

[JDDev0]

This is the maximum position you can get the scrollbar thumb with the mouse:

When resizing the window the scroll bar might overshoot this position (This is an issue with the scrollbar itself and not with the example):

[ickshonpe]

When resizing the window the scroll bar might overshoot this position (This is an issue with the scrollbar itself and not with the example):

Oh yeah that looks bad, is that bevy_ui_widgets scrollbar?

It looks like underlying the problem is that update_scrollbar_thumb uses the value of the ScrollPosition component to calculate the position of the thumb, instead of the clamped scroll position from computed node.

[JDDev0]

From my testing it only overshoots in two cases:

1. If the scroll position is set to something lower than 0.0 or higher than the maximum
2. If the window is resized and the max scroll position of the scrollable area gets smaller

edit: When creating this PR, I did not test resizing the window. I only tested the first case for which this PR fixed the problem.

[JDDev0]

Yes, this is the bevy_ui_widgets scrollbar.

[ickshonpe]

Maybe it's a change detection bug or something then, where the position isn't updated correctly when the size changes.

Part of the purpose of the examples is to help identify bugs, so I'm not sure a work around like this is the right approach. It'd be better I think if you could find a fix for whatever the underlying problem is in the scrollbar implementation itself.

[JDDev0]

I can't reproduce the problem outside of my bevy project [running on bevy v0.17.3] (I'm using a deeply nested UI node hierarchy where everything uses vmin, vw, vh for sizing, border size, padding, and margin and the font size is also automatically adapted for the current window size).

I'm trying to create a MRE with bevy v0.17.3. Afterwards I updated it to the latest commit of bevy and check if the problem still persists.

[JDDev0]

I closed this PR, because the problem is no longer present in 0.18.0-rc.2.
I think that the problem was fixed in #21378.