[BUG] - Dash 2.1 - Slider/RangeSlider - can't restrict selection to defined marks

[LiamConnors]

Thank you so much for helping improve the quality of Dash!

We do our best to catch bugs during the release process, but we rely on your help to find the ones that slip through.

Describe your context

dash                 2.1

Describe the bug
In versions of Dash prior to 2.1, it was possible to create a Slider or RangeSlider that only allowed you to select values defined with marks.

From the docs:

"If slider marks are defined and step is set to None then the slider will only be able to select values that have been predefined by the marks. Note that the default is step=1, so you must explicitly specify None to get this behavior."
With Slider and RangeSlider i

This is not possible with Dash 2.1

Expected behavior

That it would be possible to create a Slider or RangeSlider that only allowed you to select values defined with marks.

[alexcjohnson]

(Just to be clear for anyone else watching this: There's no Dash 2.1 yet, this bug refers to Dash built off the current dev branch, so an early 2.1 candidate)

[snazzybunny]

Is there a way to test Dash 2.1?

[alexcjohnson]

@snazzybunny you can build it from source, off the tip of the dev branch, following the steps in https://github.com/plotly/dash/blob/dev/CONTRIBUTING.md#getting-started