I'm not sure when this behaviour started, but I get it with the QtC snapshot from 13.04.2010 and I think the one from 10.04.

1. If I start a program and hit a breakpoint, the locals window is populated, but the watch window is hidden.
2. When I try to add a watch nothing seems to happen.
3. When I stop the debugger the watch window appears and the newly added watch is in the watches list.
4. When I start the debugger again, the watch window disappears just after the dumpers have been initialized and the breakpoint is hit. It is visible while the dumpers are getting initialized.

The way the watch is added does not seem to matter (e.g: right click variable, from the debug window popup menu, etc).