You could just have the player window (the whole thing) able to resize. Minimum size is the size of just the player data, maximum size is the size of the player/pet data.
Then set the Pet data to be a fixed distance from the top, or side depending on the layout.
Your OnShow/OnHide commands still hide/show the pet data based off if you have a pet or not.
This way, if you are a pet class you have the window stretched full to show the pet data. If your pet dies the window does not resize but the pet data goes away.
If you are a non pet class or fluff pet class you can just shrink the window to the player info, and then it doesn't matter if you have a pet or not.
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