Screen dims except navigation bar
UPDATE: Well as it often hapens, I found out solution right after posting this question. Using instructions from this post Determine which app is drawing on top of other apps? I have identified the offending package to be com.lenovo.powercenterhd which is Lenovo Power Manager, where at some point I have apparently enabled Protection Glassess feature, which dims the main screen. After turning it off, brightness went back to normal.