How do people make stretchable Widgets with animations?

Recently, I started exploring and making Widgets. So far i am into reusable, custom widgets(user created) which provide possibility for modularity and i really like it all so far.

But after completing my first widget with custom animation in it, i’ve encountered significant problem which spoils all the fun and convenience of modular user widgets. I am talking about fixed, only manually specified values for animations:
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In my example, my animation is Moving visual item, from left to right. That simple. However, my widgets can’t be stretched/scaled because of that anymore, forcing all the things in the layout to be put on Size Boxes or something like that.

After two days of trying to find a solution, or hints/advices for proper widget creation and animation practices i found nothing related to this issue.

Any advices and suggestions will be appreciated. Thank you.

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