Try disabling Fullscreen Optimizations (FSO). This issue can occur, for example, when you Alt + Tab.
How to disable FSO for Age of Empires II: Definitive Edition
- Locate your game folder (default:
C:\Program Files (x86)\Steam\steamapps\common\AoE2DE). - Right-click AoE2DE_s.exe and choose Properties.
- Open the Compatibility tab and check Disable fullscreen optimizations.
- Click Apply, then launch Age of Empires II again.
For the original discussion, see:
You can also disable fullscreen optimizations system-wide to work around this issue:
- Open Settings and go to System > Display.
- Scroll down and select Graphics.
- Under Default graphics settings, turn off Optimization for windowed games.
This will disable FSO for all titles.
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