Here's some background:
I've created a layout with Ukelele, modified the keylayout-file manually, placed it in /Library/Keyboard Layouts/mine.keylayout and I'm using it for several months so far.
Its definition starts with
```
<keyboard group="0" id="-1400" name="My layout" maxout="1">
<layouts>
<layout first="0" last="0" mapSet="ANSI" modifiers="commonModifiers"/>
</layouts>
...
```
I use it on High Sierra and Catalina. The annoyances I experience are that IntelliJ (jvm) does not ignore the mentioned dead key placeholder, so in IdeaVim's visual mode that placeholder gets immediately interpreted as a command/shortcut instead of waiting and treating the next (combined) key as the command.
Or when pressing a dead key inside any textfield in IntelliJ and then a key that does not form a combination, the dead key's placeholder (which is the terminator) gets repeated instead of prepended, like ^ + k doesn't result in ^k but in ^^
Yesterday I edited /Library/Keyboard Layouts/mine.keylayout with sudo vim and some time later (probably after various mac sleeps) I saw the change in behaviour, that the dead key placeholder is no longer shown.
And also noticed IntelliJ correctly recognising a dead key combination in visual mode and handling stuff like ^k as expected.
After lots of searching with google, I also found an old message from Geke in here:
https://groups.google.com/g/ukelele-users/c/mmyLa8zN3bY/m/pRwcaLnEk30Jthat would further the thought that my overwrite has something to do with it and it's probably just a bug in mac.
However, since it fixes the problems with IntelliJ (and I have a bunch of often used dead keys in my layout) I wanted to look into it.