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]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

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

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