User reported issue: fluctuations of pressure readings after moving to different tab

[oliver-phet]

Operating System: Personal Computer (Windows 7)

Browser: Google Chrome

Problem description: This sim has 4 tanks. When 3rd tank is selected and weights are placed in the left limb. The pressure readings fluctuate randomly.

Steps to reproduce & Screenshots:

1. Open sim & Select 3rd tank. Keep the pressure meter at any point in the fluid of right limb. Note the pressure reading.
   

2. Keep 500 kg weight on left side. Let the fluid settle. Note the pressure reading.
   

​ 3. Open any other tab in browser (lets say: Calculator). Click on that tab.

4. Click back to sim tab. And now check the pressure reading. You will observe that pressure reading has changed, even though you have done nothing (i.e- Position of pressure meter is not changed).
   

​
Severity: Mild for small loads (i.e 250 Kg). As more loads are added, severity level increases. More fluctuations are observed.

Few important Points:

1. It happens only for 3rd tank, when left limb is loaded with weights. In case of no load, reading works perfectly fine. No fluctuation occurs.

2. I've tested it on Mozilla and Windows 8 also. Problem persists in these operating system and browser also.

3. Fluctuation in reading happens only for first 1-3 clicks. After that reading gets stabilized.

4. Final stabilized reading (after clicks) is not repetitive. Remove all the loads from left limb. Now place them again. You will not observe same reading again in most of cases.

Troubleshooting information (do not edit):
Name: ‪Under Pressure‬
URL: https://phet.colorado.edu/sims/html/under-pressure/latest/under-pressure_en.html
Version: 1.1.6 2018-05-10 13:21:45 UTC
Features missing: touch
User Agent: Mozilla/5.0 (Windows NT 6.1; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/67.0.3396.99 Safari/537.36
Language: en-US
Window: 1366x662
Pixel Ratio: 1/1
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: {"assert":{"sha":"7d27130a","branch":"HEAD"},"axon":{"sha":"5e33541f","branch":"HEAD"},"babel":{"sha":"66a012fe","branch":"master"},"brand":{"sha":"0188d40b","branch":"HEAD"},"chipper":{"sha":"013c7ae0","branch":"HEAD"},"dot":{"sha":"bf6f038b","branch":"HEAD"},"fluid-pressure-and-flow":{"sha":"46c9948a","branch":"HEAD"},"joist":{"sha":"a5ef1b18","branch":"HEAD"},"kite":{"sha":"dce9765e","branch":"HEAD"},"phet-core":{"sha":"c48bf320","branch":"HEAD"},"phetcommon":{"sha":"d8ed75d8","branch":"HEAD"},"scenery":{"sha":"3e5b3661","branch":"HEAD"},"scenery-phet":{"sha":"436bf61d","branch":"HEAD"},"sherpa":{"sha":"ab4d5c14","branch":"HEAD"},"sun":{"sha":"9f2d40d5","branch":"HEAD"},"tandem":{"sha":"0ad35fb7","branch":"HEAD"},"under-pressure":{"sha":"f58d7fd0","branch":"HEAD"}}

[oliver-phet]

I was also able to reproduce using the 500kg weight moving between tabs on MacOS 10.13.6 on Chrome 67.0.3396.99.