Clicking on Activity Monitor's Dock icon won't maximize the window I've got an early 2011 13" MBP running OS X 10.10.4 PB, but this issue has been happening on literally every version of Yosemite, stable and public beta, since the initial PB release. You know how if an app has no active windows but some windows minimized, clicking on that app's Dock icon will open a minimized window? Well, that just doesn't happen on Activity Monitor.

I've found no other apps, Apple or third-party, that have this glitch, but clicking on Activity Monitor's Dock icon will change the menu bar to Activity Monitor but leave the app's window in the Dock. How can I fix this?
I've confirmed this behavior as well on 10.10.4 on a MacBook Air.  Drove me nuts until I came across this post confirming others are experiencing the same behavior.

Fortunately there is a workaround.

If Activity Monitor has been minimized to the Dock and you need to un-minimize it, click its Dock icon to bring the app to the foreground.  Then, to actually reveal the Activity Monitor window, press Command-1.  Command-1 is a keyboard shortcut listed in AM's "Window" menu purposed to give focus to the "Activity Monitor" window.

Alternatively, instead of minimizing AM to the Dock, you could hide it with Command-h.  Unhiding by clicking AM's icon in the Dock doesn't appear to be an issue.