Widgets stacking at the bottom when animating a PageRoute Transition - animation

Question

I'm trying to animate a screen sliding in from the right and the current screen sliding out to the left at the same time as if the screens are joined together.
I made a custom PageRouteBuilder and transition as follows:
class SlideInRightRoute extends PageRouteBuilder {
final Widget exitPage;
final Widget enterPage;
SlideInRightRoute({this.exitPage, this.enterPage})
: super(
pageBuilder: (
BuildContext context,
Animation<double> animation,
Animation<double> secondaryAnimation,
) =>
enterPage,
transitionsBuilder: (
BuildContext context,
Animation<double> animation,
Animation<double> secondaryAnimation,
Widget child,
) {
var exitBegin = Offset.zero;
var exitEnd = Offset(-1.0, 0.0);
var exitTween = Tween(begin: exitBegin, end: exitEnd);
var enterBegin = Offset(1.0, 0.0);
var enterEnd = Offset.zero;
var enterTween = Tween(begin: enterBegin, end: enterEnd);
return Stack(
children: <Widget>[
SlideTransition(
position: exitTween.animate(animation),
child: exitPage,
),
SlideTransition(
position: enterTween.animate(animation),
child: enterPage,
)
],
);
});
}
The exiting screen is
class HomePage extends StatelessWidget {
#override
Widget build(BuildContext context) {
return Scaffold(
backgroundColor: kWhite,
body: Column(
children: <Widget>[
Expanded(
flex: 38,
child: /* stack with containers, image, and icon */ ,
),
Expanded(
flex: 50,
child: /* column with text widgets */,
),
Expanded(
flex: 12,
child: /* container with text widget */,
),
],
),
);
}
}
The entering screen is just a scaffold with text.
When I trigger the page route, it's almost perfect. The exiting screen slides off to the left, and the new screen slides in from the right. The only problem is that the widgets in the exiting screen all quickly drop to the bottom of the screen. When I navigate back to this screen, the widgets are briefly in the bottom, then get redrawn in the correct locations.
Why is this happening and how can I have the widgets stay where they are as the new page transitions in?
I have a feeling it is because of the expanded box widgets perhaps not knowing the size of the screen or something during the transition.
TLDR: trying an animation of a screen coming in from the left and pushing out the current screen. exiting screen widgets are redrawn at the bottom as the screen exits.
Thank you for your help.

Answer 1

Figured it out.
I was using the wrong widget when calling SlideInRightRoute.