[BUG FIX] Update slider sizes without flickering the component

[Tzook]

Once the component renders, it calculates the slider sizes and positions. It does so in a timeout, which causes the slider to be rendered and re-rendered differently - which results in a small visual flicker. The timeout is needed only for actual resize events, so this behavior is kept. When the component mounts or updates, it does immediately without a timeout - this makes the component much more smooth and render beautifully.

[mpowaga]

Thanks for submitting this @Tzook! I needed to apply these changes in another commit (mainly to solve conflicts and improve the code slightly) but added your attribution.