Using VS2005 SP1. This started when I applied SP1
The way I have been able to reproduce it is as follows:
Form Designer is on one vertical tab group, code is on another
Code has focus, FD window does not
Single-click on any control in the FD tab group and the default event for the control is created (aka double click behavior)
If I click on the tab group the FD is on, then single click the control, no code is automatically generated.
This is real annoying because the new method is created at the end of the code, and I'm often checking the code associated with a control then have to go scroll back to place I was in. Defeats the whole point of having two monitors

.
Hopefully someone at MS can fix this in the next service pack.