not sure what you mean “open activity from widget/notification”
but after delete cache of QuickStep i try from notification
- open AccuBattery and AFWall
- open “update avaiable” message from GooglePlay
- open Bluetooth settings from Bluetooth Tile
- open some apps (from Nova Launcher Docks and Apps)
and still now not problem as before 
about “interfering” of QuickStep, this as i write above,
because QuickStep is not only stock launcher, bul contains “Card Switcher”
aka “Recents Apps” via com.android.quickstep.RecentsActivity,
so this part of QuickStep is what you see if activated as you write “multitasking pane”