SUMMARY In edit mode, you can open the widget settings, but edit mode closes automatically, which is inconvenient when you're customizing your desktop and only want to make minor adjustments to a widget.
This was actually done intentionally; you can see https://bugs.kde.org/show_bug.cgi?id=488429 for the bug report about it, and the merge request implementing it. Generally speaking, it would be best to always avoid windows being open in edit mode, as the two can interfere.