feat: add disabling "selection" for parent node fast-tree-item's with child nodes.

[vogtn]

🙋 Feature Request

We would like to support a tree-view with parent tree-items that are not selectable, but is still able to open and close, w3, to access the child elements and select them instead.

🤔 Expected Behavior

From https://explore.fast.design/components/fast-tree-view

<fast-tree-view>
    <fast-tree-item selectable="false">Tree item 1<fast-tree-item slot="item">Tree item 1 - 1</fast-tree-item>
    </fast-tree-item>
    <fast-tree-item selectable="false">Tree item 2</fast-tree-item>
</fast-tree-view>

😯 Current Behavior

Disabled/Selectable currently affects entire tree.

💁 Possible Solution

Selectable? attribute

🔦 Context

Our implementation example is a "teams-channel-picker", we have teams as the parents, and teams's channels as the child nodes. Teams aren't selectable, but the channels are. See the mgt storybook teams-channel-picker for example functionality.

Disabled functionality doesn't support this, as we still want the child nodes selectable.

💻 Examples

Sample, red is not selectable:

[chrisdholt]

I've updated this issue title to better represent what I think the ask is. From what I've understood from our conversations, currently parent nodes within a tree item (items with child nodes) are selectable and interactive. The one and primary concern I have here (as mentioned offline) is that I'm not sure that can be done in a way that is both standard based and accessible, and so I'm not sure it is something we can for sure add first-party support for.

With that said, I'd like to take the following approach here:

1. Investigate if this is a standard pattern.

2. If this is a standard pattern, we can support it. If it's not a standard pattern, is there something we are doing which makes you taking this approach prohibitive AND is that something we can find a way to enable this.

The above leaves room for first party support while at the very least identifying what's preventing it and seeing if there is a way we can enable you accomplishing your goals with this scenario.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[janechu]

Unfortunately @microsoft/fast-components has been deprecated for some time.