First (with HDMI connected, in multi-display environment):
Open System Preferences > Mission Control > uncheck “Open Displays as separate spaces.”
Log out and log back in. Any luck? If not...
Second:
Start up in Safe Mode. Among other things, this purges certain system caches, which may be corrupted, and subsequently rebuilds them.
Shut down your machine, and then restart it while holding Shift at the chime (let go after two seconds). Log into your user account. Log out. Restart normally. See if the issue persists.
Third:
Log into your user account
Click the Finder Icon and press the key combination ShiftCommandG to bring up a “Go to Folder” Window.
Select the following text
. Copy it, then paste it into the window ~/Library/Preferences/com.apple.finder.plist
. Move the file to your trash. Log out, then back in. Finder preferences will be reverted to defaults.
If it’s still happening, repeat (3) for the following:
~/Library/Preferences/com.apple.dock.plist
~/Library/Preferences/Byhost/
. Inside this folder, trash any com.apple.finder*
file.
~/Library/Preferences/Byhost/
. Inside this folder, trash any com.apple.dock*
file.
Fourth:
if necessary...