Scarlett and Melly's relationship has improved immensely since Melly's turbulent arrival a year and a half ago, but they regress periodically and have very physical fights punctuated by growling and hissing. Melly is usually the instigator, but Scarlett is so sensitive that it doesn't take much.
Here, Melly decided she wanted to be on the window sill, which resulted in Scarlett growling loudly. Eventually Melly decided that the cat tree was perfectly fine.
Since these two eat out of one dish together, hunt spiders together, and slurp bathtub drips together, I'm not too worried about the fighting.