(Voicing Toolbar) White strip under "chevron" button

[zepumph]

While investigating phetsims/ratio-and-proportion#378, @jessegreenberg and I found that there is a bit of an issue with the space directly under the chevron button for the voicing toolbar.

It doesn't overlap with the visibleBoundsProperty, so nothing in the ScreenView overlaps with it. @jessegreenberg told me that was purposeful. As a result, if a sim set's its background based on that Property, which is meant to be the only visible space in the sim at that time, then you get a strip where you can just see the Sim's (or screens?) background color. In Ratio and Proportion that means a white strip:

In John Travoltage we found that the floor and wall are specifically made much larger than the visibleBoundsProperty, but given a small enough scale, you can still reproduce it

Some possible solutions:

• Make all sims rely on the backgroundColorProperty for this part of the visuals, so it looks seamless. I COULD do this is Ratio and Proportion, but would rather not.
• Make this space part of the visibleBoundsProperty for a screen. This would potentially allow for overlap with the button and content in the layout bounds. Likely this sounds like a gotcha waiting to happen.
• Somehow create an appropriate NOTE somewhere that discusses this and makes people code around it by expanding background content well beyond the visibleBoundsProperty.

Over to @jessegreenberg to discuss with designers. Please report back over in phetsims/ratio-and-proportion#363.

[zepumph]

I ended up dilating this to support Ratio and Proportion, but with a TODO pointing to this issue to determine fix this once we have a designed decision about how to proceed here.