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This mixin calls the "scrolled" method of some object with no parameters, so there is no way that consumers would ever call `event.preventDefault()`. Therefore we can make the listeners passive, and improve scrolling performance on mobile.

This commit also updates the mixin to remove JQuery usage. The API is slightly modified to remove the need for an event 'name' for binding/unbinding.

The calls to `.bindScrolling` and `.unbindScrolling` in user-stream.js are removed because they are already called by the LoadMore mixin which is applied to the component.

The `bindScrolling` method claimed to offer debouncing-by-default. However, a bug in the `opts` parsing meant that debouncing was skipped if a 'name' was passed in. Therefore the only consumer actually being debounced was the LoadMore mixin. This commit fixes the opts parsing, so all consumers get the same behavior.

However, when scrolling, debounce is rarely what we want. The documentation of `bindScrolling` says "called every 100ms". In fact, debounce means that the functions were only called 'after the user **stops scrolling** for 100ms'. If you're scrolling very slowly (e.g. when using momentum-based scrolling on mobile), then this can be quite frustrating. This is why "Load more" is only triggered on topics/topic-lists when you completely stop scrolling.

Therefore, this commit also replaces the default 'debounce' with a 'throttle'. The 'throttle' is configured with `immediate = false`, so that it fires on the trailing edge, and therefore the final call will always be **after** we finish scrolling. (the default `immediate: true` would fire on the leading edge, and so the last call could be up to 100ms **before** we finish scrolling).


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