Bottom screen in state wrapper picks up thermometers

[KatieWoe]

For phetsims/qa#560. Occurs in previous RCs. (Sorry)
It is possible to drag an object over a thermometer on the first screen and have it behave properly in the top screen (thermometer left behind) and improperly on the bottom screen (thermometer picked up and carried with object). This can happen even if thermometers are in their box. Can be done with a mouse, does not require touch.
Seen on Mac 10.15 Chrome

Edit: Reset all does not solve this. Dragging the thermometer in the top screen moves them in front of the object that picked them up, but does not lent you move the thermometers.

[kathy-phet]

This seems odd to find in the state wrapper. It's pretty obscure, but I'm also surprised that it happens at all.

[jbphet]

I've examined the code and set some breakpoints, and I can see why this is happening. Code was added to StickyTemperatureAndColorSensor to make it stick to any object that it is over when state setting completes. That's because the actual state of ElementFollower instance, which is the thing that makes the temperature sensor 'stick' to an object, isn't actually being included in the state.

The code in question looks like this:

    // Check if any sensors should start following an element after being set by state
    Tandem.PHET_IO_ENABLED && phet.phetio.phetioEngine.phetioStateEngine.stateSetEmitter.addListener( () => {
      followElements();
    } );

[jbphet]

This code causes or is related to another problem, one in which once a thermometer is stuck to a model element in the downstream sim, it can't get un-stuck. Here's a screen capture, observe how the thermometer in the lower frame stays on the block once it's there, even though the color indicator associated with it changes.

[jbphet]

Here is another problematic sequence of events - notice how the thermometer doesn't stick in the top frame but does in the bottom:

[jbphet]

The problem here, in a nutshell, is that the code that implements the "sticking" of the thermometers to the blocks and beakers has not been instrumented, so its state is not always being correctly set in the downstream sim, which leads to some incorrect "sticking" behavior. I've added code to turn off the element follower (i.e. the thing that makes the thermometer stick to - aka follow - a block or beaker) at the beginning of state setting process and then decide whether it needs to be turned back on at the end of state setting. This appears to work for all of the cases described above.

A better solution would be to convert ElementFollower to a form that works better for instrumenting and then instrument it so that its state is included in the phet-io state information. This is more complicated, which is why I didn't do it. I discussed this with @zepumph and he's up for trying that conversion on master. I'll create an issue for this and assign it to him.

@KatieWoe - for now, please test this on master an let me know if it's looking good. That will give us all confidence that it will be fixed in the next RC.

[KatieWoe]

This issue looks fixed, but energy chunks being added was buggy when they were being added to an object on master. I don't know if/how this may be related. Haven't seen in the RC so far.

[KatieWoe]

Ok, it does look ok on master now. Not sure what caused earlier. I must not have had a good copy of master.

[jbphet]

It may be that some of the work done in #391 improved things, since it was all about energy chunks that were on their way to something but not there yet.

It's good to hear that it's working now. I'll mark it as fixed and will have it tested in the next RC.

[phet-steele]

Looks fixed to me. 👍