Instant messages, if they are too long to display properly on your screen, will be cut and not fully displayed. If the message on your screen ends with an ellipsis, (...), you should go to your NavBar and select Message Log to see what you missed on the display. Additionally, on some machines, the popup box clearly goes off the bottom of the screen. If it does, again, check your message log.
At the bottom of this way-too-long message is an ellipsis... (Chrome on Mac)

Example of way-too-long message running off the bottom of the screen... (Chrome on PC)

Click on Message Log to see the rest of the message or additional messages...

At the bottom of this way-too-long message is an ellipsis... (Chrome on Mac)
Example of way-too-long message running off the bottom of the screen... (Chrome on PC)
Click on Message Log to see the rest of the message or additional messages...