Windows 10 - System Tray Icons Won't Hide

EDIT 21-03-2023: While setting up a new computer I found that, in my case, this setting was the result of a Group Policy on our network. In Group Policy navigate to User Configuration > Administrative Templates > Start Menu and Taskbar. Check if "Turn off notification area cleanup." is configured. Remove it to get your notification extension area back!

I discovered the solution by trial and error / accident.

Google-fu suggested that this registry key:

HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer

was related to numerous Tray related problems so I looked there and found a REG_DWORD entry labelled:

NoAutoTrayNotify

which was set to 1. I changed it to 0 and restarted Explorer et voila my system tray starts working again. I've no idea how this got set in the first place, but I'm happy now anyway.

This link tells us "This setting determines whether the items are always expanded or always collapsed." which sort of suggests it should be controlled by the Win10 option "Always show all icons in the notification area", but toggling that option had no effect for me - even with Explorer restarts.

A new "sneaky" feature of 1809 (1803?) allows one to hide icons into the area above the up-arrow thus:
Drag them from the taskbar and hold them over the up arrow, whereupon the window opens for the icon to be popped in a location of one's choice.

I was having a similar problem. The up arrow was there, but it did nothing. The date and Action Center were gone. A few other icons I have set to "always on" were still there. I have no idea what caused my system to do that. This is the first time it happened. I have not installed anything lately, nor applied any system updates. I only noticed this when I tried to pair a new bluetooth headset. I keep the bluetooth icon in the Hidden Icons popup window which would not display.

I found an answer that worked on another forum. It dated from 2018 but it worked for me in Windows 10.

-Go to the Task manager Processes tab
-Find Explorer
-Select it and click Restart

It took a minute for that to complete, but my task bar notification area is back to normal.

Oddly though, when I went back to the Task Manager later, the Explorer process was not listed. Haven't rebooted since then, so it's possible there are side effects of this method.