Currently, Full Screen actually triggers the native full-screen that takes over all monitors. I very often find myself using Shift-Option-Maximize on windows to make them take up the entire screen without entering the native full-screen mode.
If none of these words are actual terminology that's meaningful (sorry!), here's the difference I mean. Fullscreen currently does the same thing as the green dot here:

I would love to see an additional option to do the Shift-Option variant of that same green dot.
Thank you for the work you put into this app, I love it 鉂わ笍
I think you might actually be using the macOS full screen keyboard shortcut and triggering the macOS full screen functionality, with cmd-ctrl-f. Spectacle's "Fullscreen" shortcut is not the macOS fullscreen. It is "set window to visible frame of screen", and it defaults to cmd-alt-f. That's not identical functionality to the "alt + green button" that you mention, but I think it's what you're looking for.
At any rate, Spectacle hasn't been maintained in a couple years. I rewrote it in Swift as a new app called Rectangle, and you're welcome to try it out.
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I think you might actually be using the macOS full screen keyboard shortcut and triggering the macOS full screen functionality, with cmd-ctrl-f. Spectacle's "Fullscreen" shortcut is not the macOS fullscreen. It is "set window to visible frame of screen", and it defaults to cmd-alt-f. That's not identical functionality to the "alt + green button" that you mention, but I think it's what you're looking for.
At any rate, Spectacle hasn't been maintained in a couple years. I rewrote it in Swift as a new app called Rectangle, and you're welcome to try it out.