NSLayoutconstraint created programmatically fails

Question

I am on Xcode 8.3.3, OSX not iOS.

I have a customView I use as container to replace other views inside it. The dark blue box on the right represents the container:

The constraints of the container are set in IB as follows where "scrollView" is the tables scroll view left of it:

This enables vertical scaling (scrollView is allowed to scale vertically) which is the desired behaviour:

Now I add a subview to the container with this code:

NSStoryboard *storyBoard = [NSStoryboard storyboardWithName:@"Main" bundle:nil];
    _previewViewController = [storyBoard instantiateControllerWithIdentifier:@"showSHPreviewViewController"];

CGRect newFrame = CGRectMake(0, 0,CGRectGetWidth(_previewViewController.view.bounds),CGRectGetHeight(_previewViewController.view.bounds));
_previewViewController.view.frame = newFrame;

[self.previewContainer addSubview:_previewViewController.view];

This works as expected:

My Problem is, I want the subview to be stretched so it fits the whole height of its container. Therefore I have to set NSLayoutConstraints programmatically. But i don't get the logic.... Whatever I try leads to a behaviour where neither the scroll view nor the container can scale vertically at all.

This was my last try before I wrote this post:

[self.previewContainer addConstraint:[NSLayoutConstraint
    constraintWithItem:_previewViewController.view
    attribute:NSLayoutAttributeTop
    relatedBy:NSLayoutRelationEqual
    toItem:self.previewContainer
    attribute:NSLayoutAttributeTop
    multiplier:1.0
    constant:0]];

[self.previewContainer addConstraint:[NSLayoutConstraint constraintWithItem:_previewViewController.view
    attribute:NSLayoutAttributeLeft
    relatedBy:NSLayoutRelationEqual
    toItem:self.previewContainer
    attribute:NSLayoutAttributeLeft
    multiplier:1.0
    constant:0]];

[self.previewContainer addConstraint:[NSLayoutConstraint constraintWithItem:_previewViewController.view
    attribute:NSLayoutAttributeBottom
    relatedBy:NSLayoutRelationEqual
    toItem:self.previewContainer
    attribute:NSLayoutAttributeBottom
    multiplier:1.0
    constant:0]];

I can confirm that the content of the added subview is setup correctly and resizeable (no "height-lock").