Peek definition (Alt+F12) no longer does anything in Visual Studio

One cause of this is Nvidia GeForce Experience intercepting Alt presses while listening for Alt+Z, which is the activation toggle for Nvidia ShadowPlay.

I fixed this by opening GeForce Experience and going Settings -> Share -> Off.

(Activating screen recording is left as an exercise for the reader.)

In GeForce Experience, remove the Alt+F12 keyboard mapping for Toggle FPS Counter on/off found in:

Settings -> General -> In-Game Overlay -> Settings -> Keyboard Shortcuts

This fixes this issue without disabling the whole In-Game Overlay feature.

Had similar issue and the fix was simply resetting the hotkey Alt+F12 to peek in Visual Studio with following steps:

1. Go to Tools->Options->Environment->Keyboard
2. Select (or search) the key Edit.PeekDefinition and click on Remove
3. Inside the textbox near Assign button press Alt+F12
4. Finish by pressing Assign and OK buttons

Here is an image for steps just in case you need:

PS. This solution also worked when Intel Graphics was opening on Alt+F12