The box code will stretch a flexible object as it sees fit. In the above example the box is shrinking, and despite the specification of 1000 pixels as the preferred height, the button, because it is flexible, shrinks with the box. It continues to shrink until the minimum required height for the button is reached. After that, the button can shrink no further. If the box were to continue to shrink, the button's contents would be clipped, and a portion of the button would no longer be visible.
Components can therefore have their own notions of minimum and maximum intrinsic sizes.
For example, the titled button was smart enough to know that it could not get any smaller than the minimum height required to draw its borders and its text. When specified, they provide extra information to the box as it shrinks and grows the object in question. In a horizontal box, for example, if a minimum width is specified, then the flexible object will not be allowed to shrink below that width. In the first screenshot the box has been shrunk until it is smaller than 100 pixels in height. Because the iframe has a specified minimum height of 100 pixels, it is not allowed to shrink any further, and so as the box falls below 100 pixels, the iframe gets clipped, and portions of it become invisible.