betweensystempadding breaks opus in header in 2.6

[Stephen]

In this example, betweensystempadding = -1\mm affects the spacing between 
composer and opus in the header, causing them to overlap whien using 
Lilypond 2.6.

I do not think the spacing between systems should affect the spacing between 
lines in the header. Has this bug already been caught?

Stephen

headerex.ly
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