Cocoa Autolayout - Why can not I remove or modify the width constraint (purple) on a text field?

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I'm building a simple application using autolayout, and I've run into a strange situation. I place a Text Field in an empty part of a large open view so it's not affected by anything but the super view, but when I try to modify the "Width" constraint to be >= instead of ==, it creates a new constraint and refuses to modify the old one. I can't delete it, or change any of its attributes, because it just creates a new one.

Here is a comparison of the two constraints, the purple one being the stubborn one, and the blue one being the newly created one.

Why is the purple rounded one not modifiable?


I have worked around the presence of undeletable-but-unwanted constraints in IB by setting their priority to 1. Doesn't seem like the Right Thing to do, but sometimes I'm not smart enough to be a Cocoa developer.