I am writing code that links against an LGPL library that I've modified. What is the scope of LGPL in such case? Will I have to make my code public under LGPL?
Is it possible to keep my source code proprietary still? I have no issues with making the modified third-party library public.
You can keep your code proprietary, but you must release the modifications you made to the LGPL library.
LGPL means, for libraries, you can include them in your projects (as linked libraries) without having to [L]GPL your whole code-base.
The only code you have to open up is direct changes to the library source.
You have to comply with the terms of the LGPL: dynamically linking to the library, and releasing the source code of the library with your modifications.
There may be a problem if your modifications add new APIs to expose internal state or functionality which the original contained but kept protected. You may well still be in compliance with the text of the LGPL, but the hue and cry on slashdot and LWN if you get caught will be a painful public relations mess.