Pasting into IDE from a different web tab

Software Web Editor
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(deleted)

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If you use the right click menu to copy/paste, the Arduino Web Editor has the unexpected behavior you discovered. However, if you use the Ctrl+V keyboard shortcut to paste, it will work as expected.

The keyboard shortcut is much faster than the right click menu anyway.

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For future reference, there is a forum section dedicated to the Arduino Web Editor (AKA Arduino Create):
http://forum.arduino.cc/index.php?board=101.0
where people knowledgeable on this topic are more likely to see your post.