mouseMove seems to fire only in "Edit" mode, not in "Run" mode

create a materialField, call it “myField”

create a Label field

set the script of the Label field to

on mouseMove
   set the text of widget "myField" to the milliseconds
end mouseMove

In Edit mode move the mouse over the Label field.
Note that the milliseconds are shown in widget “myField”

Switch to Run mode and move the mouse over the Label Field
Note that the milliseconds are not shown in widget “myField”

Thank you for the clear recipe to reproduce this. It has been filed as an issue and our team will work to get it fixed as soon as possible.