Silent screen reader, moving focus
The dialog hides the rest of the page from assistive technology by marking the page root as hidden from the accessibility tree. Screen reader users stop hearing the background, which is exactly what was wanted.
Tab still walks straight into it. Focus lands on buttons behind the dialog, and the user hears nothing while the focus ring moves somewhere they cannot perceive.
Boundaries, outermost first: Behind the dialog: Background link, Background button (FAILED: focusable), an empty slot for the applied to the background Outside every boundary: Dialog (open on top), Keyboard user (pressing Tab), Screen reader (announces nothing) Connections: Keyboard user calls Dialog (step 1) Keyboard user must NOT reach Background button — Tab reaches it applied to the background controls Background button (step 2) applied to the background controls Background link Dialog controls Screen reader — nothing announced