Update cheatsheets

xephyr

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+---
+syntax: markdown
+tags: [tldr, common]
+source: https://github.com/tldr-pages/tldr.git
+---
+# Xephyr
+
+> A nested X server that runs as an X application.
+> More information: <https://manned.org/xserver-xephyr>.
+
+- Create a black window with display ID ":2":
+
+`Xephyr -br -ac -noreset -screen {{800x600}} {{:2}}`
+
+- Start an X application on the new screen:
+
+`DISPLAY=:2 {{command_name}}`