See the attached screenshot. Ubuntu allows the user to drag and drop multiple displays to any physical arrangement. My system uses two external displays on a shelf with the laptop on the desk under the displays. When the IDE is run under Ubuntu Linux (17,04) with multiple displays (3), there are several situations where a dialog box extends off the screen supposedly to the display below the one where I have the Arduino IDE running. If the IDE is running full-screen or to the left side of the upper left display, the dialog box or menu extends into an area below the display that is invalid. I have to reduce the size of the IDE window and drag it into the right side of that screen so that the menu or dialog box extends to an area that the lower display (built-in) is using. The suggested improvement is to either have the IDE be completely aware of the positions of the displays or to keep the dialog box or menu from running off the bottom of the display as if it was running on a single display.
I suspect very few of the Arduino target audience use Linux.
And I suspect very few of those who do will push the IDE far enough to run into the problem you mention.
I use Linux all the time but I use the Geany editor for all my programming - Arduino, Python, etc.