Hand editing in Apply Patch / Differences Viewer?

Hey folks,

We're working on a particularly difficult port from one version of a codebase to another, and we recently switched to PHPStorm. So far so good.

Part of the refactor involves hand-applying diffs (patches) one file at a time.  Up til now this involved running 'patch' on the command line and dealing with any reject files it produced in the editor.  However, I just discovered VCS > Apply Patch in PHPStorm.

When I select my diff file in Apply Patch, it works, and if I double-click the source file in question, I get the Differences Viewer popping up.  That's awesome!  But both sides say 'read only' and I can't choose what to apply, hand-edit anything, or deal with problems directly.  I really need to use it like that.  What's the best way to do so?

BTW, the diff files we have were generated as cumulative diffs from a branch in an entirely separate repo with many other changes which we DON'T want to apply or even consider, so diffing/comparing files from one of the codebases to the other is not going to work for us.

Apologies in advance if this is clear as mud, it's very complex, I mainly just need to be able to edit in the Differences Viewer and make manual decisions in the context of applying a patch.