switching between desktops loses last focused window -
  focus goes nowhere - even if there is only 1 window. 
Has to use Alt-Tab (or whatevr that shortcut is) to get back some focus. 
Pressing alt-F4 offers me quit/reboot/... i.e. closing the session.
Probably goes to the desktop behind.

when focus is on the desktop of one workspace, switching workspaces does not restore (or just loses) their own focuses - it says on the (sticky?) desktop. 
This is very dangerous as whatever u press goes to the desktop (e.g. delete will delete some shortcut/icon)

to reproduce, needs "focus follow mouse" disabled.

Also, above thing happens too if i send some window to another workspace; 
then at switching back into the old workspace, the focus is nowhere (or maybe at the desktop?) .
Doesnot matter if switching is by keyboard or by, say, clicking the 
pager-in-the-panel.

a workspace should _always_ have something focused - be it the desktop at least. Having all unfocused makes no much sense - not a usable state - unless there's really nothing to focus on 
e.g. (pseudocode):
setFocus() {
 if None == focus: focus = last/prev non-desktop focusable-item inside the workspace
 if None == focus: focus = desktop
 // None == focus here means there's _nothing_ to focus on. so i guess the window manager has the focus?
}

-----
its probably done like the clientRaise( None) below.

./events.c: handleKeyPress():  460:        
..
            case KEY_MOVE_WORKSPACE_1:
            ...
            case KEY_MOVE_WORKSPACE_12:
                if (key - KEY_MOVE_WORKSPACE_1 < screen_info->workspace_count)
                {
                    clientRaise (c, None);
                    workspaceSwitch (screen_info, key - KEY_MOVE_WORKSPACE_1, c,
                }
                break;