Rectangle is a window management app based on Spectacle, written in Swift.
Rectangle arbitrarily supports macOS v10.12+. If you're willing to test on earlier versions of macOS, this can be updated.
The default keyboard shortcuts are based on Spectacle, but there is an alternative set of defaults based on the Magnet app. This can be enabled by setting "alternateDefaultShortcuts" to true in NSUserDefaults for Rectangle with the following Terminal command:
defaults write com.knollsoft.Rectangle alternateDefaultShortcuts -bool true
Then restart the Rectangle app.
Differences with Spectacle
Spectacle used it's own keyboard shortcut recorder, while Rectangle uses MASShortcut, a well maintained open source library for shortcut recording in macOS apps. This cuts down dramatically on the number of bugs that were only in Spectacle because of the custom shortcut recorder.
There are still a number of Spectacle features that are not implemented in Rectangle, such as next/previous thirds. Feel free to send a pull request for anything missing.