Calculating listbox height in a nested gallery

Hello everyone,

I am trying to create a 3 level tree view filter. I use a gallery (parent) for the first level, a nested gallery (child) for the second level and a list box inside the child gallery for the third level.

Both the parent and the child gallery are flexible gallery. I need the child gallery with flexible height because the list box can contain any number of entries.

I need help with calculating the height of the child gallery so it will show all entries. I only want a scrollbar for the parent gallery.

Appreciate any help!