Idea Summary
let there be an SYS_CONTEXT('APEX$SESSION','APP_PAGE_ID')

Use Case
same idea as SYS_CONTEXT('APEX$SESSION','APP_ID')

Preferred Solution (Optional)
SYS_CONTEXT('APEX$SESSION','APP_PAGE_ID') does not return NULL

i know i can use apex_application.g_flow_step_id but there is no reason why APP_PAGE_ID is not available whereas APP_ID is.