in a beta version of the IDE there was a bookmark function where you could press F3 to jump through the bookmarks in the code. There was a vertical colum in front of the code where a user could set the bookmark by clicking with the mouse. I hope this function will one day be incorporated in the official Arduino IDE release but for now my question is, is there a beta of the latest 1.8.12 where that function is built-in ?
No. The Arduino developers don't seem to have been merging the development work from the master branch of the Arduino IDE into the beta branch recently. It's a lot of work to resolve merge conflicts. Probably that time would be better spent testing and merging the improvements from the beta branch into the master branch instead so we can finally get rid of that beta branch.
So the only option would be for you to merge the bookmark feature from the beta branch source code into the master branch source code, then build the Arduino IDE from source, following these instructions: https://github.com/arduino/Arduino/wiki/Building-Arduino
Perhaps we'll get a more official answer to your question here: https://github.com/arduino/Arduino/issues/9471#issuecomment-586535604
Here's how I do it. Where you want a bookmark, type in a text piece that's not used anywhere in the code. I use "tink" as an example, commented out of course. Then type your code word into the Find window. Hit the "Find" button repeatedly and you will see the place you've marked with the code word pop up one by one.
It's actually easier to do this than to use, say, Microchip's bookmark function.
Thanks for sharing your technique @Adam912!
To provide an update to my last reply, I should say that only three days after I wrote that Arduino’s developers had merged all the improvements from Arduino 1.8.12 into the beta build.