After upgrading to Mac OS X 10.10 Yosemite, I did Show Web Inspector in Safari 8. It opened in pane, but I refer window so I pressed the small window icon. Web Inspector pane went away (it actually flickered for a moment) and window never opened. Now Web Inspector will not open at all. I tried both menu and keyboard shortcut. I also disabled and then re-enabled Developer menu from preferences.
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