Calculate total user engagement on mobile

I want to calculate the total user engagement (session duration) done by a user in a particular mobile session. We also have events from mobile to track if a user background or foreground the app. The background and foreground events are tracked when the user moves the app to background (the app goes off screen) and foreground (the app becomes visible on the screen again).

Session Table:

Expected output:

Explanation: The time between first time background the app from the open the app is 6 seconds (2023-12-05T05:40:28.096+0000 minus 2023-12-05T05:40:22.000+0000).
The time between subsequent background the app from the last foreground the app is 11 seconds (2023-12-05T06:18:42.202+0000 minus 2023-12-05T06:18:31.955+0000). so 6 + 11 = 17 seconds.
And so on.

I want to write sql query to calculate session duration. Any sql language will work, I just want to understand the logic.

Based on the answer provided by Rob and Nick, I refer and combine them and created the below:

WITH time_idle AS (
    SELECT
    sessionid,
    SUM(datediff(
        second,
        prev_timestamp,
        collector_tstamp)
    ) as time_idle_duration
    FROM
    (
      SELECT
          sessionid,
          event_name,
          collector_tstamp,
          LAG(event_name) OVER (ORDER BY collector_tstamp) AS prev_event,
          LAG(collector_tstamp) OVER (ORDER BY collector_tstamp) AS prev_timestamp
      FROM EVENTS
      WHERE event_name IN ('application_foreground', 'application_background')
    )
    WHERE event_name = 'application_foreground' and prev_event = 'application_background'
    group by sessionid
)
select
    DISTINCT
    sce.sessionid,
    datediff(
        second,
        MIN(sce.collector_tstamp) OVER (PARTITION BY sce.sessionid),
        MAX(sce.collector_tstamp) OVER (PARTITION BY sce.sessionid)
        )
    - ti.time_idle_duration as total_duration,
    datediff(
        second,
        MIN(sce.collector_tstamp) OVER (PARTITION BY sce.sessionid),
        MAX(sce.collector_tstamp) OVER (PARTITION BY sce.sessionid)
        ) AS absolute_time_duration,
      ti.time_idle_duration
from EVENTS sce
JOIN time_idle ti on sce.sessionid = ti.sessionid

I am just calculating total down time by a user on a session, and subtract with total duration of a session.