The solution for this problem is just by doing these simple steps:
1. Open the Warcraft 3, and as usual it will minimized to tray automatically.
2. Click and hold Warcraft 3 tab on the tray.
3. Right-click your mouse and hold.
4. Release your left-click (sure your right-click still in hold) and Warcraft 3 is on-view.
5. Wait a few seconds, then release the right click.
If you alt + tab and this problem comes again, just simply follow these steps again. I hope this guidance will help you somehow. Thank you for reading.