Alpha scroller for RGBA does not synchronize with mouse movement

# Current Condition :

When we click and drag on the Opacity scroller in `SketchPicker`, the scroller does not have a good position.

The scroller's position does not placed in the middle of the mouse cursor.


# Expected condition

The scroller should follow the exact position of the cursor.

Start by locating the SketchPicker implementation and its RGBA opacity scroller, then reproduce the click-and-drag behavior described in the issue. The work is done when the scroller follows the mouse cursor with its position centered correctly during dragging.

Written by the indexing model from the issue text.