Not sure what's going on there but messageBox() is throwing an error for some reason (use CTRL-J in Acrobat to open the Console):
GeneralError: Operation failed.
XFAObject.messageBox:3:XFA:form1[0]:subMain[0]:Table1[0]:Row1[1]:TextField1[0]:ready
Argument mismatch in property or function argument
It works if you add some text:
xfa.host.messageBox("Row Number: " + thisRow);
app.alert() works: app.alert(thisRow);
And console.println() works (less annoying for testing than popups - open the Console window to see the results):
console.println(thisRow);
Ah, just figured it out - messageBox() doesn't seem to like a number first, it's expecting a string. The following works with your original script:
var thisRow = this.parent.index.toString();