Perhaps the most universal yet easy-to-maintan approach is this:

Make your top-level visible UI element a Grid, it should be arranged by rows. Define <Grid.RowDefinitions> for it. All rows should have Height="Auto" except one, that one needs to have Height="*".

Your variable-height element should be placed in this * row. This way, it will occupy all the remaining vertical space. This space will be controlled by the size of your entire window.

You can have more than one row with Height="*", but then you should better place a <GridSplitter< between other UI elements to be vertically resized, such as borders. The approach can be easily generalized for the combination of rows and columns at different levels of nesting, and some of them can also be of variable height or width.

Please try out and let us know if you face any problems with that.

Thank you.