You are correct that the range sliders are fidgety as I have experienced this issue with the example project as well. It was happening well before the transparency update and the new dynamic mouse transparency didn’t seem to change the way it bugs out. The entire browser widget is just very buggy when it comes to drag-n-drop operations so there hasn’t been a way to address this issue directly in the engine.
However in terms of a workaround there are a few things you can do. The easiest is to just not use range sliders or other weird input elements at all and just use JavaScript for everything. We actually use the jQuery UI for our form elements including range sliders and it removed a lot of hassle. Otherwise another potential workaround if you absolutely need to support native input elements could be to implement some kind of mouse position tracker in JavaScript which would manually remove focus from the slider if the mouse moves too far away. This could be achieved by manually triggering the mouseup event on the input element. But with that being said the jQuery UI is probably your best option with the fewest headaches.