when viewing snapshots, duplicate headers are sometimes displayed

repro:

in firefox, open a snapshot, e.g. this one
close the tab
undo the close tab (on Mac, cmd+shift+T)

expected:

the snapshot looks just as it did before it was closed

observed:

the header gets duplicated:

misc notes:

• I first became aware of this from a user who seemed to be getting this pretty regularly through "normal" snapshot usage, without any kind of tab-unclosing. I haven't found a repro without tab-closing.
• The duplicate headers are, indeed, the manifestation of nested iframes. Apparently something's going funky with the ?content=true:
  
• I was also able to repro this in Chrome with the same steps, with one difference: In Chrome, if I refresh the tab, the extra header goes away.
• In Firefox, every time I refresh with cmd+R, another header is added!
• However, if I refresh with cmd+shift+R (refresh and ignore browser cache), it goes back to the desired state with just one header, which points pretty strongly at some unexpected behavior with browser caching.
• Maybe this is as simple as setting some cache control headers?

Start by reproducing the snapshot URL in Firefox and Chrome, then inspect the nested iframe behavior around ?content=true during tab restore and refresh. The fix is complete when reopening or refreshing a snapshot consistently leaves one header instead of duplicating it.