MONSTR

MONSTR from White Elephant Audio provides up to six bands of stereo width control to correct mix issues or use for creative effect.

Compatible with AUv3 hosts.

Also available (for free) on Windows, macOS, and Linux, as a VST3 and AudioUnit: https://www.whiteelephantaudio.com/plugins/monstr

MONSTR is an open source project released under the GPL. You can find the source code here: https://github.com/jd-13/MONSTR-Stereo-Imaging