Yes; I originally intended to display a few buttons on the top bar if the screen happened to be too narrow to show the leftmost panel, but kinda forgot to implement that . Currently, it hides the left panel, but does not display its supposed replacement in that situation; the lack of those may make the tool unusable if the screen happens to be too narrow. :P
A workaround until that is fixed would be to either increase the browser's window size, or reduce the browser's zoom level until the leftmost panel appears; this leftmost panel is required int order to access the sprite editors, the map editor and the project import/export functionality.