I found some time and the urge to learn some javascript, it's not as bad as I thought (I last tried some years ago, so maybe it was worse then). I added some commits which should fix the problems I wrote about in https://github.com/tchapi/Adafruit-GFX-Font-Customiser/pull/17
Please note that in 53dd090 I also fixed a bug I introduced in cc98f26, so you can't cherry-pick cc98f26 without fixing apppend -> append.
I found some time and the urge to learn some javascript, it's not as bad as I thought (I last tried some years ago, so maybe it was worse then). I added some commits which should fix the problems I wrote about in https://github.com/tchapi/Adafruit-GFX-Font-Customiser/pull/17
Please note that in 53dd090 I also fixed a bug I introduced in cc98f26, so you can't cherry-pick cc98f26 without fixing apppend -> append.