I know 8.5.3 is obsolete but I know there are users (like me) who revere this version so I thought I'd mention a few issues that occur when working with two files open.
 
1) I have double-clicking a Clip set to display Staff View but if I'm working in the second file I opened, Sonar switches files and opens the Staff View belonging to the first file opened. I then have to minimize the first file and click the Staff tab under the Synth Rack area in the second file to see the Staff View. No fix but not a big deal.
 
2) Is a bigger deal. File > Info is a text area for entering info about the song. If two files are open and you edit text in one of the file's Info panels, you could lose all data in the other file's Info panel. That's happened to me more than once. Sonar behaves as if the Info panel is a shared call when two files are open. I once lost all the lyrics to a song. The only way I know to avoid this is to edit the Info panel for a song when only that file is open.
 
3) Cursor stops moving smoothly and moves with a stuttering motion.
Until recently, I was able to reset the cursor movement by pressing the Pause key on the computer keyboard. Today is the first time that hasn't worked. 
 
rev
 

I still have 8.5 installed on a couple of my computers and haven't opened it now for a long time. I only kept it going so I could help trouble shoot for 8.5 users on the forum. That has now seemed to have died completely. Pretty rare you see a thread about it and my guess the few you do see as new users are people who have pirated it.   I hung on loyally to 8.5 through x1 and x2 but finally caved with X3. 
At first I really hated the damage done but now I would never even think of going way back there. The things you can do with the latest Sonar I could not live without now. 8.5 is possibly 1/4 of the program we now have. 
I will always have a soft spot for that version as I spent a good 6 years with it and it served me well. 
 
Since  x3 we have had no time to ever get comfortable with any version. Hang on to your hat..here comes another update!! For the rest of my ruddy life too!  I'll be 85 years old when I will owe them more money :)