On my Windows computer, all I have to do is press the letter D and the reference window goes away and I'm back in the regular Develop mode.
Might sound like an odd thing to suggest but have you tried to remove that reference image? You can always add it back in. Right click > go to folder in library > select that image and remove it from the catalog (not delete from disk)...
JimHess - I was already in the regular develop mode so that didn't do anything.
cmgap - I doubt that would have helped considering it chooses new reference photos whenever I go to a new directory.
@Hal P Anderson - Clicking the icon just takes you to loupe mode, which I was already in so that made no difference. However, resetting the preferences did the trick. Rather drastic and I hope I never have to do that again but it works, thank goodness. Thanks!
Replying to this one directly since it didn't let me @ you in my other comment. Clicking the icon doesn't do anything when you're already in loupe mode but resetting the preferences fixed it so marking your answer as correct. Thanks!
Edit: Well, I guess this forum isn't threaded so it doesn't show up under the other comment and looks a little odd but hopefully it at least notified you.
No problem. Glad to help.
I am having a similar problem (Adobe Photoshop Lightroom Classic CC 7.4 Release). After trying the reference photo option, a reference window won't go away. Pressing the D key does nothing, clicking on the Loupe button does nothing, deleting the preferences file does nothing.
If I select a photo (#4) in Library mode this is what I see:
so far so good, now if I go into the Develop module, this is what I see:
the display is hosed. The reference window never goes away.
I have Loupe selected but that little window wont go away:
The 'floating' image could mean you possibly have a problem with the Video graphics.
Remove the tick from [Use Graphics Processor] in the Performance tab of the Preference dialog. (& Re-start LR)
Update the Graphics Card Driver from the manufacturer's website.
I would not expect that pressing D would do anything as you are already IN the Develop module to be using "Reference View"
Ok, no go. Unchecking the Graphics Processor didn't fix it. I uninstalled and reinstalled and now the reference view is gone, but the map and web modules have a floating window that wont go away (even with the graphics processor unchecked):
In addition, the tree view of my catalog is totally gone and if I move my mouse up and down over the left hand windows (like the Filter Collections) the icons disappear and then re-appear randomly.
LR is pretty much unusable now.
Your 'floating' windows look very similar to using a second monitor, but with only one monitor in use.
Checking the  button gives me a floating window- but I have menus and controls available.
Have you tried clicking on the second monitor button?
Otherwise I would still advise-
Re-install Lightroom- But first do a full 'Clean' of Adobe files by using the Adobe Creative Cloud Cleaner Tool. This often does a better job before you re-install from the CC Desktop App. Use the Creative Cloud Cleaner Tool to solve installation problems
You havn't stated your Operating System! Have you tried updating or re-installing the video card driver software?
Oh, sorry. Windows 10. I checked the second monitor but its not that. I re-installed the graphics drivers but no-go. I reverted back to the drivers from Jan 2018 but that didn't work either. I re-installed LR but no-go. Strangely enough, if I select the Windows 8 Compatibility Mode for LR, the floating windows go away, but the tree view icons on the left panel appear and disappear randomly when the mouse cursor passes over them. I hadn't used LR for a few weeks so I'm not sure if it was the latest LR update or a Windows update that is causing the problems.
You certainly have some weirdness happening!
I am now out of suggestions, but more questions-
Did you run the Adobe CC Cleaner Tool, or did you just do a 'Windows' un-install, before re-installing?
Can you post the top 10 lines from- LR Menu: Help > System Info...
And you pay a monthly subscription for Creative Cloud?