This would be even better if it used reserved characters instead of brackets, so you could also use it as a normal font. You could even use different characters to start different QR code sizes. E.g. type 'ctrl+shift+u 11 enter' to start the QR code and 'ctrl+shift+u 14 enter' to end it, but for larger QR codes enter 12 or 13, instead of 11. (Or maybe you'd need to change the end character, to select the QR code size.)
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This would be even better if it used reserved characters instead of brackets, so you could also use it as a normal font. You could even use different characters to start different QR code sizes. E.g. type 'ctrl+shift+u 11 enter' to start the QR code and 'ctrl+shift+u 14 enter' to end it, but for larger QR codes enter 12 or 13, instead of 11. (Or maybe you'd need to change the end character, to select the QR code size.)