In the other Git GUIs that I'm familiar with, such as GitX and GitExtensions, double-clicking a file in the working tree pane moves it to the index pane. (And double-clicking a file in the index pane moves it back to the working tree pane.)

In SourceTree, I keep inadvertently opening XCode because my muscle memory tells me to double-click a filename. I don't want to open XCode. I don't use it and it's slow to start up.

Please consider changing the behavior of double-clicking a filename, or at least making it configurable.