Reminds me of Powersnap on the Amiga. Many applications did their own text rendering without supporting cut and paste, and so this guy called Nico Francois had the bright idea of letting you select a region of a window, and matching the standard fonts against the windows bitmap.
Of course then it was "easy": almost all the text would have been rendered with one of a tiny number of fonts available on the system, with little to no distortion.
Powersnap was amazing. I seem to recall it was usually able to figure out what font each program was using and only had to search for letters for that specific font, and only fall back to a bigger search if that failed. I might be misremembering, but regardless, it was essentially as fast as any copy-paste today, in an environment where many programs weren't even written to support it.
Even though it solved a problem we don't usually have today (this story notwithstanding), it was still one of the most amazingly useful programs ever.
You're probably right - the manual says it did. It'd be able to get the last used font from the RastPort structure used to draw to the window [1].
If the window was rendered with multiple font that wouldn't be reliable, but I guess it'd likely be "good enough" to avoid a wider search most of the time.
Of course then it was "easy": almost all the text would have been rendered with one of a tiny number of fonts available on the system, with little to no distortion.