Allow only initial selection for Nav

[mturoci]

Is your feature request related to a problem? Please describe

A tour.py could improve its UX by preselecting nav item based on active hash.

Describe the solution you'd like

Introduce initial_value attr on ui.nav_card.

Describe alternatives you've considered

Tried to use value, but this allows app developers to manually control which nav item is currently active (clicking on other items does not work out of the box unless handled in python code via value). However, I only care about the initial selection without further overhead.

[lo5]

The initial_value / value duo introduces a new kind of behavior that might need to be applied to all such components going forward.

It would be good to solve this problem without introducing the initial_value attribute.

I don't think there is a scenario where the app author will want to control the value on every click. It's more likely that the app author intends the value to be an initial value, and then have the nav behave normally for subsequent clicks.

What do you think about:

• When a value is specified, we use that to initialize the nav, and store it in an internal initialValue variable.
• When an item is clicked, we use that as the active item (ignore the value attribute).
• If the app author changes the value explicitly later, we can detect that by comparing state.value with our initialValue variable, and use that to set the active item.

[mturoci]

Brilliant proposal @lo5!

I don't think there is a scenario where the app author will want to control the value on every click. It's more likely that the app author intends the value to be an initial value, and then have the nav behave normally for subsequent clicks.

I 100% agree, however, it seems like there can be some people needing that - see #283.

I went through all the components using value and it seems like we are not consistent in this behavior. Some components use it for initial value only and some for full control. I suggest applying your proposal and unify all the components to have the same behavior. However, note this might be a breaking change for some apps unfortunately.

[mturoci]

Part of #1154