Hmm, not sure what's causing this (particularly the disappearing elements upon mouse movement). A couple of things to try:
(1) Replace the existing <trial Consent> with the following alternate definition:
<trial Consent>
/inputdevice = mouse
/stimulusframes = [1 = Consent, agree, disagree]
/isvalidresponse = [trial.Consent.response == "agree" || trial.Consent.response == "disagree"]
/monkeyresponse = ("agree")
/ontrialend = [if (trial.Consent.response == "disagree") script.abort()]
</trial>
(2) Theorizing that the problem is somehow related to the HTML rendering, either temporarily remove the <html Consent> element from the script and <trial>, i.e.
<trial Consent>
/inputdevice = mouse
/stimulusframes = [1 = agree, disagree]
...
</trial>
or replace it with some other type of stimulus element (<text> or <picture>) for testing purposes:
<picture Consent>
/ items = ("SomeImageDisplayingSomeConsentInformation.jpg")
/position = (50%, 40%)
/ size = (70%, 70%)
</picture>
Let me know if any of that affects or changes the odd behavior you've observed with the original script. Thanks!