It's more of a cure of symptoms rather than a cause, but these uBlock Origin filters work for me. They hide the signup popup. There is also a filer list called EasyList Cookie[0], which hides the cookies popup.

  twitter.com##div#layers div[data-testid="sheetDialog"]:upward(div[role="group"][tabindex="0"])
  twitter.com##html:style(overflow: auto !important;)

0: https://github.com/easylist/easylist/tree/master/easylist_co...