Detect when NSWindow's "Show Tab Bar" gets enabled

Question

I have a height-limited window: It cannot grow above a certain height (based on its current content), but can be made smaller (in which case I'll add a vertical scrollbar).

I limit the maximum height with a NSLayoutConstraint.

Now, if the user enables tabbed windowing by checking the menu command View -> Show Tab Bar, the window's contents get moved down in order to make room for the tab bar but the entire window's height remains the same, which effectively leads to the content being "squished", so that my current code decides to add the vertical scrollbar, as if the user just reduced the window height manually.

I rather have the window grow with the bar tab instead. How can I accomplish this?

There seems to be no event or notification that would inform me when the Tab Bar got enabled.

What's a clean way to detect the activation of the Tab Bar then, so that I can grow my window height along with it?

Answer 1

I poked around with KVO and found that I could watch NSWindow's tabbedWindows property for this purpose (tested on 10.13.6).

In the ViewController's viewWillAppear:

if ([self.view.window respondsToSelector:@selector(tabbedWindows)]) {
    [self.view.window addObserver:self forKeyPath:@"tabbedWindows" options:0 context:nil];
}

And then handle the change:

-(void)observeValueForKeyPath:(NSString*)keyPath ofObject:(id)object change:(id)change context:(void*)context {
    // ... adjust my window's height
}

This may not be reliable, though, i.e. I found no documentation that guarantees that this will work in the future and is not just an accidental side effect.