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Jeff Garbers - PRODJeff Garbers - PROD 

Unable to modify Apex classes from Lightning component in Developer Console after installation

I recently installed a Lightning unmanaged package I've been working on into our production environment. Now, when trying to work on the package through Developer Console in a scratch org, I can no longer make changes to Apex classes used in that package. When I edit and try to save one, the error message says "managed installed classes cannot be saved".

I am able to edit and save Lightning components from the Console with no problem. Also, using Salesforce DX, I am able to edit Apex class files locally and use sfdx force:source:push to upload the changes to the scratch org. Using push lets me get work done, but I'd still like to make quick experimental changes in Developer Console.

Can anyone explain what's happening here and offer a fix? Thanks!