Wow, that's the answer. Thanks a lot to mdom!
And of course, in retrospect, it's all documented. I opened LS Preferences, then went to Help > Little Snitch Help > Analyze Traffic > Network Monitor > Toolbar, and there it was. The little equal-sign-like thing in the right end of the tool bar is called the Floating Button, which makes the monitor "float" onto whatever space is selected. As I should have known from all my years as a software engineer: RTFM.
And of course, in retrospect, it's all documented. I opened LS Preferences, then went to Help > Little Snitch Help > Analyze Traffic > Network Monitor > Toolbar, and there it was. The little equal-sign-like thing in the right end of the tool bar is called the Floating Button, which makes the monitor "float" onto whatever space is selected. As I should have known from all my years as a software engineer: RTFM.