This is in Radar as <rdar://problem/4988325>
1. Load the attached example page, which is a reduced version of the one in radar 4960095/bugzilla 12449
2. Command-P to bring up Print dialog
3. Click Preview button
Usually the assertion failure happens here. But sometimes, I have to repeat steps 2 and 3. I've never had to repeat them more than once to hit the assertion failure, in many attempts, so I'm marking this repro always.
On a release build the printed page shows overlapping, doubly-drawn text, and the web page redraws to show that same bad appearance. See the attached screenshot. There don't seem to be any other symptoms on a release build -- reloading takes the page back to normal, and subsequent pages aren't messed up.
Created attachment 13094 [details]
Test case used to demonstrate the problem.
Created attachment 13095 [details]
Screenshot of overlapping text
Screenshot that shows the results after printing, on a Release build (where the assertion doesn't fire)
Created attachment 13099 [details]
Reduction (does not require printing)
The bug is that a style recalc resets the isInline() flag of the run-in to false.
See also bug 12522.