AngularJS $watch inexplicably changing value of the watched value

I'm trying to implement an animdated version of ng-show and ng-hide; I originally tried to use jQueryUI.toggle('slide', …), but since $watch fires multiple times, my elements were toggling in and then immediately toggling out (sometimes more than once). But I saw in AngularJS's github issues that that is $watch's intended behaviour (dirty checking).

So I thought, okay this little eff-you be damned, I'll explicitly show or hide instead of the simple toggle: I broke it down to check the value of $watch's newValue like so:

scope.$watch(condition, function myShowHideAction(newValue,oldValue) {
    if ( newValue !== oldValue ) {
        if (newValue) {
            elm.show("slide", { direction: direction }, "slow");
        } else {
            elm.hide("slide", { direction: direction }, "slow");
        }
    }
}

Fire it up, and what do I see? Toggling!

After putting in a few console logs, I discover somehow my condition's value is being changed during $watch's loop (which iterates like 6 times instead of just once, but that should be neither here nor there). However, the actual scope parameter's value does not change (as it shouldn't) half way thru like newValue's does.

What the heck is going on?

Plunker