Many browsers have recently introduced Picture In Picture feature. You can pop out a video and place it on a corner on screen while you do other things.
If you're using i3 window manager, here are the lines you need to add in your i3 config to make Picture In Picture browser window available on all workspaces :
for_window [title="Picture-in-Picture"] sticky enable
Sticky windows, as the name suggest will make the window stuck on screen whichever workspace you go to.
You can also make any other windows stick on your screen like this. For this you need to first make a window
floating and then make it
Add these lines to your i3 config file :
bindsym $mod+Shift+space floating toggle bindsym $mod+Shift+S sticky enable
Go to the window, say VLC media player, press the key combo
Meta+Shift+Space first (this will make window float) and then
Meta+Shift+S. Now the window will be sticky. You can then resize the window to the size you need with
Meta+R. Drag to move the window to the position of your choice !
VLC & other players have a
compact mode which removes the interface and plays the video in full width available. You can use that to fit the video completely in the floating+sticky window. For SMPlayer, compact mode can be toggled with