OK .. we will check this. Your are right that the the focus in BLOB viewer should not be lost when switching away from and back to application with BLOB viewer open.
Issue confirmed. When using “Insert/Update Blob Field” editor if you navigate to another application using ALT+TAB and then come back to SQLyog, the entire text is selected and the cursor position scrolls back to the top. We will look for a possible fix. It is added here: http://code.google.com/p/sqlyog/issues/detail?id=1559