If the option "Open new file in subwindow" is disabled, the file menu should offer "Open in subwindow" in addition to "Open". So you have the chance to work with tabs even if the option is disabled in the settings.
Why not just enable tabs?
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This bug has been in NEEDSINFO status with no change for at least 30 days. The bug is now closed as RESOLVED > WORKSFORME due to lack of needed information. For more information about our bug triaging procedures please read the wiki located here: https://community.kde.org/Guidelines_and_HOWTOs/Bug_triaging Thank you for helping us make KDE software even better for everyone!