After updating to Windows Phone 7.5, have you noticed the on-screen keyboard randomly collapsing itself as if you had pressed the back button? Some users (such as Branislav Hupka) believe that this is due to a specific line of code that may have made it into certain apps by accident. The code: “ScheduledActionService.LaunchForTest” is used only for testing purposes and should not be included in the final release of your Mango-compatible app. A developer named Docteur G has posted the below video demonstrating how the keyboard disappears at exactly the time when a background agent starts to run. If you’re noticing a lot of keyboard disappearing issues or miss-presses, try going to Settings > Applications > Background tasks and turning off the background agents for all of the apps you have installed. It may be that only certain background agents are causing the problem, so feel free to turn on one at a time to see if everything is okay.
This explanation certainly makes sense to me since I had been using Mango since June and haven’t seen these issues until recently after installing apps with new background agents (since they didn’t exist until recently). Hopefully 3rd party developers can easily fix this within their own apps and/or Microsoft can fix it in a quick update.