This bug has been bothering me since day 1, where sometimes the toolbar icons (particularly drop-down lists) or even parts of the UI vanish while working. Research has told me that this is because max 2015 is using hardware acceleration to draw the UI and in complicated scenes or when lots of graphic's intensive resources are being called up, some parts of the UI don't get completely redrawn.
Either way, an interesting workaround was mentioned on one of the forums (sorry, I don't remember the user). Pressing the "M" key to quickly toggle the Materials Editor will force a redraw of the icons and UI. Seems to be working so far. Thanks! (whoever you were).