How to click and drag to select/copy only part of a message in iMessage for OS X? Can anyone tell me how to restore the feature that allows clicking and dragging in a message in the OS X Messages app to select only part of a message that has already been sent or received?

This capability seems to come and go. I just did it a few days ago, and now it doesn't work again.
On a whim, I tried quitting Messages, and re-starting just the app, and it actually worked; before quitting I was unable to highlight part of a message. Afterwards, now I can select a subset of a message. I can also double-click to select a single word, etc.