Combobox aria-activedescendant should update as you arrow up/down the list, not after you commit the change

[metapzl]

Bug Report

• Package version(s): 5.50

Priorities and help requested (not applicable if asking question):

Are you willing to submit a PR to fix? Yes

Requested priority: Normal

Products/sites affected: Anyone using Combobox for accessibility

Describe the issue:

Open Combobox, arrow key up/down. It is easier to see the problem when you use Narrator on Edge.

Actual behavior:

The blue bounding box of Narrator stays in the text input box and doesn't follow the dropdown's arrow keys. Also, Narrator does not read the 'selected' item.

Expected behavior:

Narrator's blue box should follow the arrow keys, and Narrator should naturally read the 'selected' item.

If applicable, please provide a codepen repro:

Just use the Fabric example site.

[metapzl]

My proposed fix changes

 aria-activedescendant={ (isOpen && (selectedIndex as number) >= 0 ? (id + '-list' + selectedIndex) : null) }

into

aria-activedescendant={
          isOpen &&
          (this.state.currentPendingValueValidIndex as number) >= 0
            ? id + "-list" + this.state.currentPendingValueValidIndex
            : null
        }

[amyngu]

@jspurlin if you don't have bandwidth to fix this, I can go ahead and take it.

@dzearing for FYI

[jspurlin]

@amyngu , yes feel free to make the fix for this

[jspurlin]

This was fixed by PR: #3738