ok I get it but with attaching it to Android it can be serious product and not just a developer tools.
TStringList is from classes unit i guess?It's RTL. To decompile RTL libraries FPC 2.7.1 or later is needed.You can also try to add RTL sources to SDK's source path. (in File -> Project Structure -> SDKs).
classes.pp is from /rtl/unixBut it's almost entirely in included classes.inc so adding it in source will not help much.For FPC RTL the only way is to use a recent FPC version.However I can see no point in using older versions.
After upgrading FPC you need to recreate SDK in File -> Project Structure -> SDKs.After creating SDK with version 3.1.1, make sure that it is used as Module SDK in dependencies of your IDEA module.It will take some time to reindex the new files.If classes is still highlighted with red, use File => Invalidate caches / Restart to clear cache and restart the IDE.Also I recommend to download latest version of the plugin from site.
Kuci, can you please elaborate what means completion doesn't work?There is a problem with resolving methods of a class which is parent of a class from RTL (TStringList seems to be the case). This will be fixed.Methods and fields from TStringList itself should be recognized and included in completion.