One particular challenge I faced while designing system-level UI elements was implementing a progress bar that needed to maintain its state even when the user navigated away from the screen. The scenario involved a progress bar displaying the installation time for a software update. For instance, if the installation time was set to two minutes, the progress bar needed to fill from 0 to 100% within that timeframe. However, if the user pressed the back button, the fragment containing the progress bar would enter the onStop state, causing the progress bar to halt.

To overcome this, I utilized the `CountDownTimer` provided by Android. I implemented logic to calculate the elapsed time and the remaining time whenever the user navigated away from and back to the fragment. Here's how I approached it:

1. **Track Elapsed Time:** When the progress bar started, I tracked the elapsed time using the `CountDownTimer`. For example, if the progress bar ran for one minute before the user pressed the back button, I recorded that one minute.

2. **Handle Fragment Lifecycle:** When the fragment entered the onStop state, I recorded the system time to capture how long the user was away from the screen.

3. **Resume Progress Bar:** Upon returning to the fragment (onResume state), I calculated the total elapsed time by adding the time the user was away to the previously recorded elapsed time. For example, if the user was away for 20 seconds, I added that to the one minute, resulting in one minute and 20 seconds.

4. **Continue Progress Bar:** I then resumed the progress bar from the calculated time, ensuring it continued smoothly from where it left off.

This approach ensured that the progress bar appeared to run continuously, providing a seamless user experience even when navigating away from the screen. It was a challenging task, but it significantly improved the application's usability and reliability.