That's very curious looking.
I've never seen this before.
As lylejk has mentioned,
Where did you download Gimp and what version?
Also,
this could be a User Profile issue.
Most of the time,
if you create a new Gimp User Profile.
This can correct many issues within Gimp's interface.
You can create a new Gimp User Profile,
by renaming the current profile.
For example,
if I were to rename my current profile.
I would rename it something like,
.gimp-2.8_old.
Once you rename your current user profile.
Start Gimp and it will create a new Gimp User Profile.
You can then see if this behavior continues with the newly created profile.
If it doesn't,
then it was something within the Old profile,
which was causing the issue.
Begin to move your resources back,
a little at a time.
From the Old profile,
into the New profile.
Restart Gimp,
after each resource move.
To see if the behavior reappears.
By moving the Old resources back into the newly created Gimp User Profile,
in small batches.
You begin the process of elimination.
To determine what was causing the issue.
If this was a very new Gimp User Profile,
with little to no resources added.
You can start fresh,
using this newly created profile.
If not,
you may need to use the,
"process of elimination",
described above.