I guess there are at least a few ways this could be done:

• Updates happen behind the dropdown as each option is selected, and then clicking away from it closes the dropdown.
• Updates don't take place as options are selected until the user clicks away from the dropdown.
• Same as above, but updates occur when the user clicks a new 'Update' button or similar, to trigger the changes.

Updates don't take place as options are selected until the user clicks away from the dropdown.

This is the most straightforward (and least workflow-breaking) of the three. Updates happening in the background would likely result in rapid flashes of the list, which would be disruptive to the eye (and I imagine to the screen reader), and waste of traffic.

In T380032#10328738, @Nardog wrote:

Updates don't take place as options are selected until the user clicks away from the dropdown.

This is the most straightforward (and least workflow-breaking) of the three. Updates happening in the background would likely result in rapid flashes of the list, which would be disruptive to the eye (and I imagine to the screen reader), and waste of traffic.

The only downside I see is that it might be confusing to select these options, and then for nothing to happen - clicking away from the dialog is an opaque requirement. As such, maybe a 'confirm' or 'update' button inside the dropdown would be helpful, to make this explicit?

The only downside I see is that it might be confusing to select these options, and then for nothing to happen - clicking away from the dialog is an opaque requirement. As such, maybe a 'confirm' or 'update' button inside the dropdown would be helpful, to make this explicit?

Makes sense. I'd prolly label it "Apply".

ETA: At that point it might make sense to make it not a popup but a modal, with everything else grayed out.