maybe u accidentally press and hold wee bit too long on back button, it cause it turn on or off the multi windows function

it's a shortcut ..off course you can always turn it on and off via setting tab but for me i found it more convenience to do it in one step..press an hold back button.

now too lazy to screen capture it and post it here ...hahaha

Basically nothing extra special process happened what so ever, most consumption on screen by 60+% followed by google service.

It also self restart like hell, maybe it because i made some tweaks here n there through audism and tweakpro

here's my update on the issues i'm having after 4.3 update.

apps random crashes - most crashes are SOLVED with manual updates / redownloading the apps, others persistent crashes could be solved with factory reset and redownloading the app

gallery lag and freeze - SOLVED by backing up the sd card, and doing a reformat on the sd card.

significant battery draining when using - 

wifi connections showing unstable -    tested the wifi problem a few times with different wifis at different location. and manage to reset my home modem a few times, and even adjusting the modem channel, to see whether its my modem problem or phone problem. seems like a phone problem to me. the wifi shows that its still connected, and running, and i might still b able to on9 with the connectivity. but notifications show "wifi is unstable" and prompt me to "auto connect to other available wifi". dint experience this problem b4 update.

conclusion: will go for a factory reset to see if it solves the other unsolved problems. but doesnt seems related with factory reset or not