Download "save as" dialog doesn't appear until first bytes of the response body are received
Hello! I am developing a web server, from which users can download large files. These files take a few seconds to generate, and cannot be generated ahead-of-time; only wh… (read more)
Hello! I am developing a web server, from which users can download large files. These files take a few seconds to generate, and cannot be generated ahead-of-time; only when a user requests them. The server therefore responds to such requests by first sending the HTTP response headers (without "Content-Length"), which can be written immediately, then generating the file, and sending the generated file as the response body.
In Chromium, this leads to the "Save As..." dialog appearing instantly, so the file can be generated & downloaded in the background while the user picks a location.
In Firefox (v132, linux x64), the "Save As..." dialog waits for the file to be generated before appearing, which is not user-friendly. During deeper testing, I discovered that Firefox appears to wait for my server to send at least 1 byte of data from the file (in the response body, after sending the HTTP response headers), before the "Save As..." dialog appears.
I have searched the Firefox settings and documentation but did not find anything that would explain this behavior. Is this a bug?