Build a color picker

• A 2D area whose horizontal axis is saturation and vertical axis is brightness (value) for the current hue; dragging a handle in it changes the colour
• A hue slider from 0 to 360 degrees that changes the base colour of the area
• Pointer dragging works with mouse and touch using pointer events and setPointerCapture, and continues when the pointer leaves the element
• A hex input and R, G and B number inputs show the current colour, accept edits, validate them, and update the area and slider when valid
• A preview swatch shows the colour, and an onChange(hex) callback fires on changes
• Conversions between HSV, RGB and hex are implemented in small pure functions, with rounding that does not make the colour jump while dragging

Only after every requirement works.

• Alpha channel with a checkerboard background
• Keyboard control with arrow keys on the area and the slider
• Saved swatches and an eyedropper using the EyeDropper API where available