tickInterval
How far apart ruler ticks should sit, as the smallest step from TICKS that draws at least minSpacingPx apart.
The unit follows the zoom rather than being fixed, so ticks stay legible from a whole hour on screen down to a fraction of a second.
Return
The chosen step, and the largest one in the ladder when even that is too close.
Parameters
minSpacingPx
How close two ticks may be drawn, in pixels. Convert FilmstripTimelineDefaults.MinTickSpacing through the local density rather than passing a fixed number, which is a different distance on every screen.