A selection in a browser can be mapped back to a corresponding location range in one or more source files that generated the selection. A selection in a source file can be mapped to the corresponding location range in the browser's DOM. The selection in the browser can be mapped to a location range in a block list generated from the DOM. The location range in the DOM's block list can be mapped to a corresponding location range in a block list generated from the fetched file received from a web server. The location range in the fetched file's block list can be mapped to a particular location in the particular source file responsible for generating the selection in the browser. The selection in the browser and the source file can be displayed concurrently, the source file scrolled to the location range corresponding to the selection in the browser.