On my widescreen PC, I often work with two windows side-by-side maximized vertically and with half the screen each; browser on one side, document on the other. This leaves Firefox with as little as 640 horizontal pixels, or as many as 840, depending on which screen I happen to be using.
Zotero does not adapt well to these spaces; the sync, toggle fullscreen, and close icons disappear off to the right, even though there should be space for them to move left. The pane on the left can be closed to give space to the other panes (NEAT!), but there's insufficient ability to move its divider to the right to make it larger. At 640 horizontal, the right pane doesn't show fully, and its divider can't be moved to the left to make it larger (although paradoxically, closing the left pane then moving the divider of the right pane to the RIGHT does sometimes bring its missing contents into view). (I am working with title/creator/date/date added columns in the middle pane, in case that makes any difference.)
It would be good to have some attention to the UI at this sort of window size; my use case must be very common. Putting an open/close control on the right panel like the one on the left might be useful. And generally allowing the user more flexibility to stretch things as required would be a big help.