Raycast Issues After Alt-Tabbing in Unity
This is one of those problems that shows up when you're trying to build something interactive and you switch away from the window to check something else, then come back and everything feels wrong. The raycast lands in the wrong place, sometimes way off center, sometimes just slightly shifted. It's frustrating because it works fine when the game is active, then suddenly breaks. When you alt-tab away from your Unity game, the game enters a paused or throttled state. Unity reduces the frame rate dramatically while the application doesn't have focus. The problem is that the mouse position values you're using for raycasting may have been captured at a different frame rate or with different delta time than what the engine is actually rendering. By the time you alt-back, the stored mouse position doesn't match where the player's cursor actually sits on screen anymore. There's also the matter of input buffering. Some operating systems buffer mouse movement events while the application is in the background. When you return, those buffered events can fire all at once, creating a jump in the recorded mouse position that throws off your raycast calculation entirely.
I spent about three days tracking down a bug where my inventory system would randomly select the wrong item whenever a player alt-tabbed and came back. The raycast was returning the correct collider but with a transform position that seemed shifted by roughly the difference between the last frame rate before tabbing out and the frame rate after returning. The item selection was completely deterministic in code, but the visual feedback was wrong because the world-to-screen conversion had used stale data.
The Fix
The most reliable approach is to invalidate or re-sample your raycast input when focus is lost and regained. Unity provides OnApplicationFocus and OnApplicationPause callbacks for exactly this purpose. Here's what the basic structure looks like: You'll want to apply this to whatever manager object handles your raycasting logic. If you're using a camera-based approach, also consider recalculating the camera's viewport to world coordinates after regaining focus, since those can drift on some systems. Another layer to this is setting Screen.lockCursor if your game uses cursor locking. When the cursor is locked, alt-tab behavior changes and the mouse position reporting becomes more consistent. It's not a universal solution, but for first-person games it eliminates a lot of the edge cases.
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Common Pitfalls
The most common mistake I see is only handling OnApplicationFocus without also considering the delta time that accumulates during the paused state. Some developers reset their input variables in OnApplicationFocus but forget that other systems may still be running with stale time values. If your raycast is part of a continuous interaction system, make sure your update loop also checks focus state before applying new input. A second issue is the interaction between resolution changes and focus loss. On Windows, alt-tabbing can trigger a resolution change if the desktop and game run at different resolutions. This shifts the coordinate space that your raycast calculations depend on. I've seen projects where the raycast was off by exactly the ratio of the resolution change after alt-tabbing. Checking Screen.width and Screen.height in your focus handler and adjusting your camera settings accordingly resolves this.
When This Approach Won't Work
This fix assumes you're working with standard Input system mouse positions. If you're using the newer Input System package instead of the legacy one, the focus callbacks still exist but the mouse position access pattern is different. You'll need to read from the new input action assets rather than calling Input.mousePosition, and the re-sampling approach stays the same but the implementation details shift. Also, this won't help if the issue is on the rendering side rather than the input side. If your raycast visually appears off-center even though the logic is correct, the problem might be with camera projection or UI scaling, which is a separate category entirely. For most projects though, handling focus state and re-sampling mouse input on regain is enough to eliminate the alt-tab raycast drift. It's not the cleanest architectural pattern, but it's practical and it works reliably across Unity versions from about 2019 onward.
