There is a GPLv3-licensed open source library on GitHub which is used by the company I work for in a central component of the business. The business does not sell software, but only a manipulation of the output of software. All software is executed by the business on machines owned by the business, not by the customers. No binaries or source code or anything is distributed to the customers except for the results.
I made minor modifications (< 30 lines) to the >50,000 lines of source code of this open source library which slightly changed this open source library's output in a way that is beneficial for downstream proprietary code that is external to the open source library, i.e. beneficial to downstream code that is not open source but is proprietary to this company,
These slight modifications to the open source library are useful to the broader public, and I would like to use the modified version of this library in later work at a different employer.
If I leave the company, I would like to make the modified copy of this open source library public. For instance, I could: fork the open source library's repository from GitHub as a public fork on my personal GitHub account, copy the code with the modified parts that I made at work, and then upload the modified code to my public fork on GitHub. To be clear, I am only asking about copying and publishing the modified copy of the GPLv3-licensed library, not the all of the code surrounding it and interacting with it.
I understand that code that you write for an employer is their property. But this code is a (few lines of) modification to a GPLv3-licensed open source library. If an employee makes a modified copy of a GPLv3-licensed open source library on the job, is that modified copy property of the employer?
Is it legal to copy, keep, re-use, and publish a modified copy of a GPLv3-licensed open source library if the modified copy was created for the employer?