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I have the "Autopin PL/SQL editors" option chosen (Preferences-->Code Editor). However, when opening a PL/SQL file for the first time in a session, it is not autopinned, and I'm finding that I inadvertently close it when I open other files. However, when I select a connection for the editor, pin it, close the file, and re-open it, it is autopinned then. Is this expected behavior?
"Autopin PL/SQL editors" option is true by default. Then, I can't reproduce your experience. I open editors (doesn't matter db or file-based) and it opens it in new tab. After opening 9th editor (Max Open Pl/SQL editors = 8) the first one is closed.