Before upgrading to High Sierra, for many years prior, it was possible to highlight text (e.g. in Chrome), drag it over another application (e.g. Preview), and then hit escape without anything happening. I'm a web developer, and I did this dozens of times daily to compare the text in the browser to the text in the design: I could simply overlay the text I drug over the design and see if sizes, letter spacing, line spacing, etc. were lining up. Really quick and really convenient.
In High Sierra, it's now shrinking down that text arbitrarily as soon as it's over a new application. I imaging this is just a visual indicator that you might be able to drop the text and you're in a new application, because I can think of no other practical reason for the text shrinking.
See the demo here:
So... is there any way to get that to stop shrinking again? Might sound nit-picky, but it was so extremely useful on a consistent basis to be able to do this. I'm fine digging into shell or editing plist files or whatever.