Safari browser window goes transparent unexpectedly

For some reason, sometimes when I hold down the Apple
"command" key to open links in new tabs, the entire
browser window goes transparent... meaning I can see
the desktop through the browser window. Anyone else
experience tihs and is there a fix for it? It's a
hassle to re-open all the tabs in a new window
because the web pages are unreadable when you can see
through the browser window.

Any help or corroberation would make me feel better.

thanks.