Scratch-vm: Mouse Down sensing blocked by Monitor

[Mr-Dave2]

Expected Behavior

When the mouse is over a monitor and the mouse button is pressed the sense block for mouse down should report true.

Actual Behavior

When the mouse is over a monitor and the mouse button is pressed the sense block for mouse down reports false.

Steps to Reproduce

Drag out a forever block .
Click on the My Variable monitor.
Put into the forever block "set My Variable to [mouse down]"
Click on the forever block.

Click on the stage - but not over the monitor- and hold down the mouse button. Then notice that the monitor says true.

Then do this while the mouse is over the monitor and notice it remains "false". But under Scratch 2 it will say True.

Operating System and Browser

Windows 7, Firefox 62.0.2

I have a test case here

And this bug affects this actual program https://beta.scratch.mit.edu/#245545359

[thisandagain]

/cc @ktbee @kchadha @paulkaplan

[thisandagain]

@ktbee We don't see this use case all that often, so this is likely to be low impact and is related to the choice to render monitors in the GUI.

[paulkaplan]

@thisandagain we could probably forward click events through the dom elements if we wanted to support this without rendering monitors in canvas

[thisandagain]

@paulkaplan Yup! I was thinking / hoping the same.