I discovered that the capacitive navigation key buttons light up by themselves when the phone senses a dark environment.
In order to do that the phone uses the frontal sensor near the Nokia logo: if you try to cover it with your finger the buttons will light up in a second.
This is a new feature in Windows 10 Mobile called one-handed mode. It's designed to help people using larger-screened phones reach the top of the display better (e.g. to open action centre). If you dislike this feature, simply don't tap-and-hold on the Windows key.
For more information about one-handed mode, see this article at Windows Central.
I recently enabled soft mode navigation bar, but UC browser and WhatsApp and many more apps crashed immediately after opening. Then I disabled the virtual bar by using the value changing 1 to 0 in
HKEY_LOCAL_MACHINE /SOFTWARE /Microsoft /Shell /NavigationBar
and created a key called SoftwareModeEnabled of type dword-integer-32bits and assigned a VALUE ...
You can swipe up from the bottom of the screen to open the navigation bar.
The start bar you are referring to is popularly known as navigation bar.
You can see the offical Microsoft document here which says,
On phones without built-in buttons, reveal the navigation bar by
putting a finger below the screen and swiping up.
Try Updating Extras. I faced same problem on my Lumia 535 when I did upgrade on Windows 10 but through official Upgrade Advisor app. This caused some bugs in extras just after upgradation but when I updated Extras from the Store app it resolved almost 95% issues. See this . Updated Touch successfully turn vibration on and off.