I had submitted a support ticket and was advised to post here.
https://support.mendix.com/hc/en-us/requests/131915 is a link to the ticket with an explanation and a video included (not sure if everyone can see that though, so I'll explain it shortly here as well):
In the (range) slider widget, the value for the slider is only visible on hover, and hovering this will also determine its position on the page. By this, I mean that it has an absolute position for which the “left” property is calculated upon hovering, thus positioning it right above the slider.
However, this tooltip is rendered separately from the widget. This means that if I would, for example, just try within the SASS to make it always visible (rather than only on hover), its absolute positioning is not relative to the widget's position but to the page. So it's more fixed to its position and remains there if you were to scroll in the page.
So this is what the widget looks like when you hover the slider:
Then, if I turn off the “hidden” class (or change the code from "display: none” to “display: block” or whatever, and I scroll in the page, it looks like this:
I would suggest the tooltip to be something that's actually included in the widget, so it can be modified and styled accordingly. Right now, it's rendered on its own in the DOM, all the way on the bottom:
I highlighted the closing tags for the body and the html document, as well as what's basically the last regular <div> of the page.
I hope my point is clear and I hope this will be taken into consideration when the widget will be improved.