I get this too, and now I'm sure Avast is causing the alt+tab, because the process logger that I've installed to track the issue reports the same thing at the exact time when it happens:
12/30/2017 11:29:28
Process: [8648] C:\Program Files\AVAST Software\Avast\setup\aswOfferTool.exe
Username/Domain: [windows user folder]/DESKTOP-R9DF2R2
CommandLine: "C:\Program Files\AVAST Software\Avast\setup\aswOfferTool.exe" -checkChrome
MD5 Hash: F54FABEB4834EAA33E53E7EECD02383E
Bitness: 32-bit
Publisher: AVAST Software
Description: Avast Offer Installation Tool
Version: 17.9.3761.0
Integrity Level: High
Signer: AVAST Software s.r.o.
System Process: False
Protected Process: False
Metro Process: False
Parent: [2620] C:\Program Files\AVAST Software\Avast\AvastSvc.exe
[Process Creation]
12/30/2017 11:29:28
Process: [8912] C:\Program Files\AVAST Software\Avast\AvastUI.exe
Username/Domain: [windows user folder]/DESKTOP-R9DF2R2
CommandLine: "C:\Program Files\AVAST Software\Avast\AvastUI.exe" --type=renderer --disable-gpu-compositing --disable-pinch --no-sandbox --primordial-pipe-token=F5C53A62102BABC965FB8E684ABAF040 --lang=en-US --lang=en-US --log-file="C:\Users\[windows user folder]\AppData\Roaming\AVAST Software\Avast\log\cef_log.txt" --log-severity=error --user-agent="Mozilla/5.0 AppleWebKit/537.36 (KHTML, like Gecko) Chrome/57.3.2987.1601 Safari/537.36 Avastium (17.9.2322)" --proxy-auto-detect --disable-webaudio --mute-audio --force-wave-audio --disable-gpu --disable-software-rasterizer --no-sandbox --disable-webgl --blacklist-accelerated-compositing --disable-accelerated-2d-canvas --disable-accelerated-compositing --disable-accelerated-layers --disable-accelerated-video-decode --blacklist-webgl --disable-bundled-ppapi-flash --disable-flash-3d --enable-aggressive-domstorage-flushing --enable-media-stream --allow-file-access-from-files=1 --pack_loading_disabled=1 --device-scale-factor=1 --num-raster-threads=2 --enable-main-frame-before-activation --content-image-texture-target=0,0,3553;0,1,3553;0,2,3553;0,3,3553;0,4,3553;0,5,3553;0,6,3553;0,7,3553;0,8,3553;0,9,3553;0,10,3553;0,11,3553;0,12,3553;0,13,3553;0,14,3553;0,15,3553;1,0,3553;1,1,3553;1,2,3553;1,3,3553;1,4,3553;1,5,3553;1,6,3553;1,7,3553;1,8,3553;1,9,3553;1,10,3553;1,11,3553;1,12,3553;1,13,3553;1,14,3553;1,15,3553;2,0,3553;2,1,3553;2,2,3553;2,3,3553;2,4,3553;2,5,3553;2,6,3553;2,7,3553;2,8,3553;2,9,3553;2,10,3553;2,11,3553;2,12,3553;2,13,3553;2,14,3553;2,15,3553;3,0,3553;3,1,3553;3,2,3553;3,3,3553;3,4,3553;3,5,3553;3,6,3553;3,7,3553;3,8,3553;3,9,3553;3,10,3553;3,11,3553;3,12,3553;3,13,3553;3,14,3553;3,15,3553 --disable-webrtc-hw-vp8-encoding --disable-gpu-compositing --service-request-channel-token=F5C53A62102BABC965FB8E684ABAF040 --renderer-client-id=3 --mojo-platform-channel-handle=3268 /prefetch:1
MD5 Hash: 7891EA436E353768BAA279317A597CA1
Bitness: 32-bit
Publisher: AVAST Software
Description: Avast Antivirus
Version: 17.9.3761.0
Integrity Level: Medium
Signer: AVAST Software s.r.o.
System Process: False
Protected Process: False
Metro Process: False
Parent: [624] C:\Program Files\AVAST Software\Avast\AvastUI.exe
Parent CommandLine: AvastUI.exe /nogui
OS: Windows 10 Home (with fall creators update) and Avast Free Antivirus
I hope this will be fixed soon.