This looks a lot like the linux focus box drawing (issue #369) a problem that that was in 1.3.x for a long time.
It came down to a default X driver bug with the line width being zero. Default drivers had this problem, and often
it would go away with more precise graphics drivers installed. Since this came up so much, we decided
to work around in 1.4.x by way of commit 4d88fb6.
@vit1251 try cloning FLTK from the git repo, and it should have that workaround fix.

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