Rereading your post again, I noticed I missed your point about "form data and other things are lost on close". This is something that I had not considered because I did not know this could happen. This feature should be an opt-in, anyway, and the appropriate warnings could be put there.

Honestly, in my short time with Tabs Outliner, I have not encountered any instance where I regretted clicking on the green x (except when I first started learning Tabs Ouliner, and did not know how to save the session :). I imagine I would continue to feel the same with this proposed feature.

Hi MZ, thanks for your feedback. About your concerns, note that this action would save as well, i.e. it should be equivalent to clicking on the green x. If one accidentally double-click on a line, all it would do is to close the tab, but not delete it. One would simply need to double click again to undo the action and reopen the tab.

In other words, the only difference between this and the green x is the activation method.

Of course this could be part of the configuration so that those who prefer single click would not receive this behaviour.