FWIW I'm pretty sure I've been seeing this same bug when opening new Firefox windows, too (Ctrl+N or Ctrl+Shift+P). I've noticed it twice today -- I can't reproduce it on-demand, but I'll see a flicker of content from some not-currently-visible window or tab, as the new window opens.
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FWIW I'm pretty sure I've been seeing this same bug when opening new Firefox windows, too (Ctrl+N or Ctrl+Shift+P) -- not just popups. I've noticed it twice today -- I can't reproduce it on-demand, but when it does happen, I'll see a flicker of content from some not-currently-visible window or tab, as the new window opens.
FWIW I'm pretty sure I've been seeing this same bug when opening new Firefox windows, too (Ctrl+N or Ctrl+Shift+P) -- not just popups. I've noticed it twice today -- I can't reproduce it on-demand, but when it does happen, I'll see a flicker of content from some not-currently-visible window or tab, as the new window opens (showing up briefly in the new window before we draw the actual Firefox UI, I think).