SmugMug-style gallery fails to change tile size between Ti and Th on resize

The SmugMug-style gallery chooses between the two thumbnail sizes (Th and Ti) based on the width of the gallery viewport. But when the browser window is resized, this thumbnail size class fails to be updated, leaving the thumbnails stuck at the size they were when the page was first loaded.

For example, here's a skinny browser window. At the bottom of the screen, note that the viewport has been assigned the class sm-gallery-viewport-1024, and the thumbnails have been correctly assigned the class sm-tiles-Ti.

Now I resize my window horizontally. Note that the viewport class has been correctly updated to sm-gallery-viewport-1680, but the thumbnail class is still set to sm-tiles-Ti, leaving the thumbnails very small.

Now I refresh the page without changing the window size. Note that this time, the thumbnails have been assigned the correct sm-tiles-Th class.