There is a bug in the output window, where the text buffer for the window is probably not being flushed correctly.
For example, I use OutputDebugString to log events like window resize. When I resize the window, I get a bunch of event log lines, except for the last one. However, when I mouse over the debugger window, the last log line appears.
Another example is writing log lines for keypresses. If I start the proces and press a key, nothing is logged in the output window. When the key is pressed for the second time, the log line for the first keypress appears in the output window. Again, mousing over the debugger makes the second log line appear as well.