Hmm.. weird, cause Shift-Home and Shift-Up does the same thing for me in everything except spine.
I just found that I can press CMD+SHIFT+HOME to select everything to the start, but in other apps i don't need to hold cmd.
Also I noticed that when I want to rename an attachment, I doubleclick, all text is selected, then I can normally press left arrow key to go to the start, but it only steps one step to the left and unselects. I guess this is caused by the engine you use for the UI (which is really cool mostly) but these small little things get in the way.. 🙁
Is this something that can be reported to the author of the UI system you're using, or is it home made? I still have problems with this :/