readerview | Firefox Reader View as an add-on for the Pale Moon web | Addon library

The problem is that you have a stack view with a fixed height that contains two views (cView and readerView) that have no intrinsic content size. You need to tell the layout engine how it should size those views to fill the remaining space in the stack view.

It works when you use a .fillEqually distribution because you are telling the layout engine to make all four views in the stack view have an equal height. That defines a height for both the cView and readerView.

When you use a .fill distribution there is no way to determine how high the cView and readerView should be. The layout is ambiguous until you add more constraints. The content priorities do nothing as those views have no intrinsic size that can be stretched or squeezed. You need to set the height of one of the views with no intrinsic size and the other will take the remaining space.

The question is how high should the collection view be? Do you want it to be the same size as the reader view or maybe some proportion of the container view?

For example, suppose your design calls for the collection view to be 25% of the height of the container view with the readerView using the remaining space (the two other views are at their natural intrinsic content size). You could add the following constraint: