The taskbar will only show thumbnails of so many opened windows or tabs for an item before the threshold is reached and will then automatically show a list afterwards for that item.
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Windows Registry Editor Version 5.00
[HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Taskband]
"NumThumbnails"=dword:00000000
to revert back to normal, delete the registry key created
src - https://www.tenforums.com/tutorials/20989-change-taskbar-thumbnail-threshold-show-list-windows-10-a.html
#win7 #win10
origin - http://www.pipiscrew.com/?p=6463 taskbar-thumbnail-threshold-to-show-list