The indenting is a minor problem, but you could avoid this by setting your editor properly (Qt Creator should have this option checked by default, "replacing tabs with spaces" that is). I'm more worried about the broken diff for mainwindow.cpp, in any case I took a quick look at the downloaded diff, once again, careful with magic numbers ! This '8' that's used in a few lines should really be declared as a const int maxHistoryItemCount or something.
It's not a code question here... it's that I personally don't like this feature (in the same way I dislike the back action menu). Having to have this, I prefer Firefox solution. Just that.