While working on an unrelated change, mthiesse@ noticed a color issue with the button captions:

When hovering over a button, the button text should transition from invisible, to white.  On a grey background, the text pixels should therefore transition from grey to white.  Instead, at the beginning of the transition, the pixels change from grey to black, then from black to white.

- The caption opacity is managed via CSS, not native.
- The problem is made readily visible by changing the button caption opacity easing time to 2 seconds, rather than it's default value.

Possible causes:
- The UI transparent background?
- Our blending function?
- Conveniently well-timed cosmic rays?

Even though this issue isn't a big problem for the text, we should figure out what's going on.  It could be a sign of a bigger problem.