qmk-firmware/docs/feature_grave_esc.md

Grave Escape

Grave Escape is a feature that allows you to share the grave key (` and ~) on the same key as Escape. When KC_GESC is used it will act as KC_ESC, unless Shift or GUI is pressed, in which case it will act as KC_GRAVE.

Key	Alias	Description
GRAVE_ESC	KC_GESC	Act as KC_ESC normally, or KC_GRAVE when GUI or Shift are held.

There are several possible key combinations this will break, among them Ctrl+Shift+Esc on Windows and Cmd+Opt+Esc on macOS. You can use these options in your config.h to work around this:

Option	Description
GRAVE_ESC_ALT_OVERRIDE	Always send Escape if Alt is pressed.
GRAVE_ESC_CTRL_OVERRIDE	Always send Escape if Ctrl is pressed.
GRAVE_ESC_GUI_OVERRIDE	Always send Escape if GUI is pressed.
GRAVE_ESC_SHIFT_OVERRIDE	Always send Escape if SHIFT is pressed.