Created attachment 298484 [details]
Simple test-case reduction.
Macintosh. macOS 10.12.2 (16C67).
Steps to reproduce:
1. Download and decompress the webkit-inspector-bug.zip attachment, which contains these files: index.html, style.css, and main.js.
2a. In WebKit, open index.html as a local file using file:.
2b. Show Web Inspector’s Resources tab.
2c. Confirm that the source code of index.html, style.css, and main.js is editable.
3a. Open a terminal and change its working directory to the decompressed directory webkit-inspector-bug/.
3b. Start a simple HTTP server at the directory, such as python -m SimpleHTTPServer, which would serve to localhost:8000.
3c. Open the index.html webpage as is served by HTTP server, e.g., localhost:8000 for the Python command above.
3d. Confirm that, while the source code of style.css is still editable, the source code of index.html and main.js is no longer editable.
Resources can still be saved to local files using ⌘S, whether or not they are normally editable or abnormally uneditable.