@Marc-Andre-Rivet All references to ?m= have been expunged from Dash for R, yet this error appears on soft reloading CSS:

I believe the cause is L35745 within dash_renderer.dev.js, which appends ?m= with the CSS modification timestamp to the request coming in to the backend from the renderer:

if (a.modified > 0) {
  var link = document.createElement('link');
  link.href = "".concat(a.url, "?m=").concat(a.modified);
  link.type = 'text/css';
  link.rel = 'stylesheet';

  this._head.appendChild(link); // Else the file was deleted.
}

Is this also related to flask.Blueprint? Or can we eliminate the concat statements now?

Until 👆 is resolved and the soft reloading test passes again, this PR will have to be held.

@Marc-Andre-Rivet I've opened an issue to track the above, since I hadn't previously: plotly/dash#1359. I can take this one on if it would be helpful.

It turns out that the underlying code which takes advantage of ?m is a bit more complex to remove based on the bug observed in plotly/dash#1363; this will likely be held until some housekeeping is done in Dash for Python as well.