Thanks for using my extension! I appreciate and have considered your suggestion, but I won't add it.

This feature would essentially only remove the step of opening a new tab of the selected page. Since this only takes two mouse clicks (click URL bar to select it, drag selection to tab area) I don't think it adds much value on its own.
I considered the possibility to clone a tab including its state, meaning for example keeping text input in fields, but I've looked into it and it isn't possible.

There is a setting in the Workarounds section to add/subtract pixels when calculating how to place and size popups.
For example: if you have a rule that sets x=0 and width=100, and use this setting to add 10 to X and subtract 20 from the width, the result simulates a 10 pixel gap on the left and right sides.

Note that this only applies when the popup is opened and placed by the extension, **it's not possible to add a real invisible border** to a window from an extension. If you use for example Windows Snap afterwards then the popup will not have a gap between itself and the other window.

Edit: Sorry, I still don't understand what you are asking for, can you clarify? This is what I refer to as "invisible border": https://i.imgur.com/3fEz63K.png (It's only a problem in Windows 10 as far as I know)
(Edit 2): Maybe I understand... Do you mean the title bar that's supposed to be to the left of the minimize/maximize/close buttons but is gone in your image? That's not something I am explicitly able to hide, and I don't want to add features that relies on bugs in order to work.

Integration with TST: It looks possible, they provide an API this add-on can use to do this. I'll have to consider whether integrating with other add-ons like this is something I want to support, but at the moment I'm leaning towards not. I've created issue #40 on my github repo to track this request.

Dragging to edges: This is a Firefox bug, I'll have to refer you to their Bugzilla for this. I did not find an existing bug when I did a quick search so you might want to create one.

This is unfortunately a limitation set by Firefox. The domain URL is shown automatically in an effort to combat phishing.
If you want this behaviour changed you can make your voice heard on this bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1296365

* I've edited my reply as I misread the proposal in that bug, my apologies.

谢谢。我不懂中文。 英语好吗？

I want to keep the add-on simple, gestures is a separate and hard problem. I suggest that you contact the developer of a mouse gesture add-on and ask them to add a popup function.

Otherwise I can add an API that another add-on can use, please ask the add-on developer to contact me in that case.

Thank you! Unfortunately maximization is out of my hands, Firefox disables it on this type of window.

If fullscreen (hotkey F11) is not what you're looking for you'll unfortunately have to resize the window manually.

Thank you for your suggestions. I have replied with my thoughts to the issues you created on the github page (#12, #13), but to summarize I don't think I will be adding these features.
(Edit: I would add #2 (maximization) but cannot since it's not possible to make popups maximized)

Thanks for the suggestion, I'll add settings for that.
(Edit: I've uploaded the new version now (1.3.1), it will be visible once it has been approved)

With my current way of handling settings it's not feasible to add a list of settings, i.e. site specific. I'll look into re-designing that part to make it possible but it may take some time.
(Edit: Now available in version 1.3.2)

It's also possible to add x/y position options but during my initial tests they were always created in the middle of the screen, so this might be added in a later release.