This appears to bee a bug with options_ui.open_in_tab
This works:
"options_ui": {
"page": "options.html",
"open_in_tab": true
},
This doesn't work:
"options_ui": {
"page": "options.html",
"open_in_tab": false
},
This also doesn't work:
"options_page": "options.html",
FWIW, options_ui.open_in_tab does not seem to be documented in the Opera specific extension documentation, but options_page is somewhat documented via example[1], and that appears to be not working.
[1] https://dev.opera.com/extensions/manifest/
Version:65.0.3467.78
Update stream:Stable
System:Mac OS X 10.15.2 64-bit