I can not reproduce that, but Windows is a constant source of "what has happened" 
Some thing to try:
Press F11 ( a couple of times) and see if it brings the Gimp window back to full display +title +info
If that fails the window size is held (for samj portable) in Gimp_3.0.4_Portable...\Preferences in the file sessionrc
It is a text file, you can have a look at it and see if the size is large. It is safe to delete sessionrc Restart gimp and it makes a new default file.
Some thing to try:
Press F11 ( a couple of times) and see if it brings the Gimp window back to full display +title +info
If that fails the window size is held (for samj portable) in Gimp_3.0.4_Portable...\Preferences in the file sessionrc
It is a text file, you can have a look at it and see if the size is large. It is safe to delete sessionrc Restart gimp and it makes a new default file.