Make opacity slider wider; lower min opacity to align tick marks
Make opacity slider wider
The default width (implicit control's own width) is too small to be usable, and also looks plainly bad. Let's at least hard-code it to be somehow wider.
Lower min opacity to 15, which also makes tick marks align at the end
Because (255-15)/30 is an integer 8, unlike (255-30)/30 which is a rational 7.5. (Divisor /30 is the TickInterval here.)