If someone created a form for the document, then there is probably some VBA code that is not allowing you to change focus away from the document. The only way to undo that would be to take out/comment out the code causing this portion.
If you want to try it, whether you want to delve into the code or not, you could probably make this change, and not worry about anything else. However, if you do, you'd need to be careful changing away from the window during some operations, possibly. If the programmer put that code in there, they may have had a good reason - then again, they may not have.
Or if it's not custom code, it could be a Word setting which I'm not aware of. I know in MS Access, you can make the form modal, and that can restrict some movement from that form.
If it's ONLY happening with this one document, then I'm pretty sure it's one of the above. However, if it's happening with other programs/documents, then I'd think it's something else altogether in your system.
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"If to err is human, then I must be some kind of human!" -Me