The Hide item in the TreeView menu hides this whole section, so that only thumbnails are shown. To make TreeView appear again, reselect the same item (now renamed Show) in the TreeView menu.
If Show Size column is checked, the size of each image is shown. Otherwise, the size is not shown and this section can a little narrower.
You can resize the TreeView section of the window. If you have deeply-nested folders, you may want to make it wider. If you make it narrower, more thumbnails can be displayed.
