Invisible Equality Piece

[KatieWoe]

Test device:
Dell Laptop
Operating System:
Win 10 (touch screen)
Browser:
chrome
Problem description:
For phetsims/qa#260, phetsims/qa#262, and phetsims/qa#261
It is possible to have a piece of a fraction object in place, but not visible. This is shown by either the equality fraction being visible, or by the equivalent number fraction showing it is there. This is done with multitouch. This seems to apply to any of the types you can build on this screen.
Steps to reproduce:

1. Go to either the intro screen or the equality lab, depending on the sim
2. Drag a piece out of the bucket with your finger
3. Hover the piece over the part of the fraction you want to add it to. Do not drop yet
4. With a second finger, use the arrows to adjust the size of the pieces. The piece in your finger will seem to vanish, but don't move your finger
5. Let go of the now invisible piece. The fractions will indicate a new piece that can't be seen

Screenshots:

Troubleshooting information (do not edit):

Name: ‪Fractions: Equality‬
URL: https://phet-dev.colorado.edu/html/fractions-equality/1.0.0-rc.1/phet/fractions-equality_en_phet.html?showPointerAreas
Version: 1.0.0-rc.1 2019-01-16 21:26:49 UTC
Features missing: touch
Flags: pixelRatioScaling
User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/71.0.3578.98 Safari/537.36
Language: en-US
Window: 1536x732
Pixel Ratio: 2.5/1
WebGL: WebGL 1.0 (OpenGL ES 2.0 Chromium)
GLSL: WebGL GLSL ES 1.0 (OpenGL ES GLSL ES 1.0 Chromium)
Vendor: WebKit (WebKit WebGL)
Vertex: attribs: 16 varying: 30 uniform: 4096
Texture: size: 16384 imageUnits: 16 (vertex: 16, combined: 32)
Max viewport: 16384x16384
OES_texture_float: true
Dependencies JSON: {}

[ariel-phet]

Nice find @KatieWoe but we will allow this behavior for 1.0 and reevaluate priority after 1.0 is published.

[KatieWoe]

This is in phetsims/qa#368