When your users are experiencing an issue of the screen shifting, leaving the menu displayed only half way, you can adjust the client screen to a "fixed" location on the Administrator.
Viewport locking is a configuration in the telnet host entry. The most common setting is 1:1. You will find this in the Administrator under the Server for Emulation Settings > Telnet Host Groups> double click your group > double click your host entry in the group:
Be sure to start a new session after turning on this feature.
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