...
When a popup appears it does not have keyboard focus which means that keyboard only/screen reader users are unable to tab to the content within the popup. Screen reader and keyboard-only users can easily become distracted and disoriented.
When a popup unexpectedly appears it does not have keyboard focus which means that screen reader users are unable to use the TAB key on the keyboard to navigate to the content within the popup. They cannot visually see the popup on the screen, unless it's automatically programmed to completely shift the focus away from the main window. However, this would be very disorienting to screen reader users.
Modal Windows
If there is no other solution than a popup use a modal window. It must be programmed in a way that allows for the following.
...