What would be useful would be some way to "snap" to exactly 1, 2, 3... rows of thumbnails, and for it to stay that way—as they're scrolled
using the mouse wheel. It doesn't seem to be a problem if a thumbnail is selected and then arrow keys are used for scrolling, or if the scroll bar is used. I suppose the problem is the mouse pointer has to be over the window if the wheel is to scroll, and then the focus is changed to the thumbnail under the pointer. But it does the same thing if the pointer is kept over the scroll bar as the wheel is used.
To illustrate the problem...The first screen shot is the beginning of a selection of about 400 images. Using the arrow keys or scroll bar to scroll, there will always be exactly for images displayed. Using my preferred mouse wheel, however, it looks like the second shot after just four clicks of the wheel.