@Eric-Singer Because my suggestion is not to limit the size of the parent but rather to limit what the layout reports as the minimum size required. minimumSize() returns the minimum amount of space the widget needs to provide so that the layout can fit all the widgets inside it.
maximumSize() is the size beyond which the layout can't reasonable grow the widgets inside.
Maximum size is not useful for this case. Your problem is that the parent widget grows too much and it grows because the minimum size of the layout grows i.e. the minimum amount of space required to lay out the widgets inside grows.
If you return a fake limit, one that is smaller than the sum of all the widgets inside, the parent will be able to keep its size despite all the children in the layout not fitting (visually they will be cropped). You could, for example, return the minimum of the base implementation's value that calculates it based on the content size and some constant value of yours, beyond which you don't want to grow.
To be honest that's pretty much what a scroll area does and I think the suggestion given by @SGaist is a better one as it already does that work for you.
Reading the documentation about customizing QSplitter, it seems indeed that there's something not right going on. You should check the but report system to see if it's something known. If not, please consider opening a new report providing a minimal compilable example reproducing the behavior (well in this case the lack of it)