Sometimes UI elements are grayed out in native dialog(the load or save dialog.

[tkbd]

It happened when I was modeling for a long time with a lot of polygons.
(About 100000 faces(polygons)...,No texture use...,and I had worked at least one hour...)

In particular my mac's memory is 8GB and Core i5 2.8GHz (It's over 5 years ago)
(And,Manipulating the models of that data size was just a bit heavy for my old mac.)