Quick answer: Your game crashes on alt-tab because it mishandles the focus change, most classically a lost graphics device in exclusive fullscreen: alt-tabbing can invalidate GPU resources, and code that uses them afterward (or doesn't recreate them on return) crashes. Other causes are unsafe pause/suspend logic on focus loss. The crash can happen alt-tabbing out (handling the loss) or back (restoring).
Alt-tab crashes are a classic, the game runs fine until the player switches away and back, then it crashes. This is a focus-handling bug, especially common with fullscreen games and graphics resources. It's worth fixing because alt-tabbing is something players do constantly, so it hits a lot of players in normal use.
Why Alt-Tab Crashes Happen
When a player alt-tabs, the OS changes the game's focus and window state, and the game has to handle that. The most common crash cause is a lost graphics device: in exclusive fullscreen especially, alt-tabbing away can cause the graphics device/context to be lost or invalidated, and if the game keeps using the now-invalid GPU resources (or doesn't recreate them on return), it crashes. Modern APIs and windowed/borderless modes mitigate this, but it remains frequent.
Other causes: pause/suspend logic on focus loss touching state unsafely, and audio or input systems mishandling the focus change. The crash may happen on alt-tab-out (handling the loss) or on alt-tab-back (restoring, where lost resources are used or recreated incorrectly). It's often mode-specific (only in fullscreen) and can be GPU-specific.
How to Diagnose It
The reproduction is usually easy, alt-tab out and back, possibly in fullscreen, possibly repeatedly, so you can often trigger it yourself. The stack trace tells you what's failing: a trace in graphics code points at the lost device / invalid resource; a trace in your pause/focus-handling code points at the focus logic.
If it only happens for some players, capture it with device context, lost-device crashes can be GPU/driver-specific. Bugnet captures the trace and device context for crashes and groups them, so alt-tab crashes (especially on certain GPUs in fullscreen) cluster with the evidence. But because they're often reproducible locally, the main value is confirming the cause and checking whether it's hardware-specific.
What to Do About It
For the lost-graphics-device case, handle the device-lost / focus-change events your graphics API provides, stop using invalidated resources and recreate them when focus is regained. Using borderless-windowed instead of exclusive fullscreen avoids exclusive-mode device loss entirely and is a common, robust fix. For pause/focus logic, make the focus-loss and focus-regain handlers safe.
See our guide on fixing a game that crashes on alt-tab for the steps. Test the full out-and-back cycle in both fullscreen and windowed modes, since the bug often only triggers in a specific mode or after multiple cycles.
Alt-tab crashes are focus-handling bugs, classically a lost graphics device in fullscreen. The trace points at graphics or focus logic; recreate resources on return, or use borderless windowed.