Answer
By default, the IEE client runs without the standard Windows border around its window (borderless mode). Unchecking 'Borderless Window' restores the normal Windows border around the client. This option exists mainly as a workaround for borderless-mode-specific UI glitches (for example, new dialog boxes occasionally rendering hidden behind the main window) — most such issues have since been fixed as bugs, but the option remains available if a similar borderless-related display problem is encountered.