So, this is annoying.

This message shows up instead of the source when selecting "view source" in Chrome on a page after posting a form.
I only found two workarounds.
1) save as, then open the saved html file to see the source
2) use developer tools to inspect the DOM
None of those worked for me as desired.
I needed to get a snippet of code generated from posted information.
I suppose I could output the code in a textarea or something, but Firefox doesn't have the same problem. Viewing the source of the page in Firefox shows what I expect.