Very silly question, won't really affect my productivity or anything, but I'm curious…
Every now and then I realize my Dock suddenly looks like this:
That is, the background excludes the left-most, Finder icon. If I restart the Dock porcess with, say:
$ sudo killall Dock
(also useful to clear MRU list on cmd-click app icon)
…it goes back to normal:
What is causing it in the first place?
My Setup
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MacBook Pro with an external monitor attached, running macOS 10.12.6 ("Sierra").
-
System Preferences/Mission Control/"Displays have separate Spaces" is unchecked (i.e., the Dock always stays on main monitor).
EDIT: I tried rearranging the icons to see if anything on the far left will trigger this or it is just finder. Curiously, as of Sierra at least, the leftmost Finder icon seems to be fixed and impossible to drag/rearrange.
Best Answer
It turns out the Continuity feature Handoff is to blame: If I open mobile Safari on my nearby iPhone, this happens:
When I close the mobile app, the left-most segment disappears and Finder is "kicked out" of the Dock, like in the question.
Perhaps I sould file a bug report with Apple, but can not check if this has been fixed in the upcoming High Sierra or not...