Values in monitors can be off by a frame

[mzgoddard]

scratchfoundation/scratch-gui#2945

Expected Behavior

Execute monitors before any threads during a step of the virtual machine.

Actual Behavior

Scratch 3 adds threads to execute the monitors. These threads will be placed behind existing threads. If those threads change the values the monitors will report, the monitor values will not match Scratch 2.

Steps to Reproduce

late-monitor-execution.zip

This Scratch 2 Project works as expected in Scratch 2 and does not in Scratch 3. To demonstrate this issue the change in values must be repeated every frame. This is operation is repeated for a second so the change is legible. (Otherwise the values would change to quickly to read on screen.