xnest - 2:1.20.4-1+deb10u9 main

Xnest is a nested X server that simply relays all its requests to another
X server, where it runs as a client. This means that it appears as another
window in your current X session. Xnest relies upon its parent X server
for font services.
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Use of the Xephyr X server instead of Xnest is recommended.
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More information about X.Org can be found at:

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This package is built from the X.org xserver module.