What is this white 'No items selected' popup box in save/open dialogs?
I figured out the answer while confirming a few behaviours of the mystery white box while writing the question. I'm posting and answering the question anyway since none of my searches turned up anything.



So it seems it's the File Preview box: if you hit space in the 'Open Files' dialog with a file selected, a similar box gives you an enlarged preview of whatever file is selected. 

In the 'Save As' dialog, however, you can't select files (clicking just copies the file name), so it will always come up as 'No items selected' any time it comes up. While you can select folders in the Save As dialog, it doesn't attempt to preview them and continues simply saying that nothing is selected.

Here's a screen grab of it functioning by previewing the screen grab of itself not functioning:



Seems like it can never show anything other than 'No items selected' in the Save As dialog.