"escape sequence highlighting" going completely bonkers

[Miles Bader]

I've recently overved very odd behavior which is kind of hard to pin
down, but basically:

I'll have a buffer with some random garbagey characters (say a comint
buffer in which I've done "cat" on some spam containing binary
characters).  Usually this is displayed as a mixture of decoded
characters and octal-escaped (by the display engine) 8-bit characters.
 The octal-escaped characters _don't_ seem to use the new
highlighting.

While the buffer is just sitting there (maybe I'm doing something
else), suddenly the display will flip to showing _all_ non-ascii
characters as octal-escaped (by the display engine) characters, only
this time they're highlighted in a shade of blue, which I assume is
the new  highlighting stuff.

The display will start flipping back and forth between these two
representations rapidly (many times per second), though I'm not
actually doing anything; I think there's another comint buffer,
undisplayed in any window, that's receiving output though.  The only
way I can stop this is to change to another buffer.

It's not consistent though, sometimes it will do this, often it won't.

I've also had a lot of problems recently with the window-start
apparently getting reset randomly while scrolling.

In both cases, I haven't yet found a reliable way to reproduce the
behavior, but it's definitely happening!

Thanks,

-Miles