1)
Can you to a few tests please.
a) Copy the text from any app you want (gedit, browser, ...).
Keep the app open, launch your synedit test, and check
b) Copy the text from another app with synedit.
Keep the app open, launch your synedit test, and check
2)
At the start of your app (or in an onButtonPress), insert
Caption := clipboard.AsText;
Does the caption get set correctly?
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Note that pasting with the middle mouse button, uses a different clipboard (PrimarySelection).
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It works fine for me, with Fedora 28 / Gnome, under VirtualBox.
No clipboard helper, so the app from which I copy, must remain open.