docs(examples): Simplify scrollbar example

[marianomarciello]

This PR simplifies the scrollbar example. Divide the function ui into multiple functions such that clippy does not throw the clippy::too_many_lines error anymore. Handle correctly the cast from usize to u16.

fix #975

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.4%. Comparing base (f4637d4) to head (c82cf6f). Report is 2 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1078 +/- ## ===================================== Coverage 89.4% 89.4% ===================================== Files 61 61 Lines 15468 15468 ===================================== Hits 13839 13839 Misses 1629 1629 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.