Why does mutating an array or object directly sometimes fail to trigger a component update when using OnPush?

With OnPush, Angular only checks whether an input's reference has changed, not whether its internal contents have changed. Directly mutating an existing array or object keeps the same reference, so Angular does not detect anything different, even though the actual data inside has changed.

// Wrong with OnPush, mutates the existing array directly, Angular does not notice the change
this.items.push(newItem);

// Correct, creates a brand new array so Angular detects the change
this.items = [...this.items, newItem];