sale_order_product_picker: quick-clicking on +1 button fails

[yajo]

For example, if you want to add 3 units and click the button 3 times quickly, chances are that only 1 or 2 of those clicks are actually handled.

I think it would make sense to throttle and defer the calls, and only when the user has stopped clicking for about 500ms, then upload the changes. Does that make sense?

Otherwise, at least it would help if the button appears as disabled while you can't use it. That visual feedback isn't the best, but at least gives a hint that you have to wait to click more.

Originally posted by @yajo in #2796 (review)