On the Actions tab, I select Mouse Enter as my trigger. Then I add a script for it.
app.alert(event.target.value)
After I click OK to get out of the Java Script Editor, the trigger changes to Mouse Up.
Why isn't the Mouse Enter trigger being excepted?
It just defaults back to the first values in the drop-down. The important thing is that your action was added to the Actions list at the bottom of the dialog. If there it says "Mouse Enter" then that's what you added... It should look like this:
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