pdf failing to close in roundcube

[telemeister]

I use roundcube on several devices and with different OS and  browsers. 
Generally excellent.

On one device (Apple ipad + safari) roundcube appears to have got stuck in a particular mode.
It is where I have opened a PDF to read. The PDF is showing and the "close" button
has no effect.

No matter if I close the browser tab etc whenever I start roundcube on that device
I get the same window with open PDF.  It basically means I can no longer read my
mail on that device.

All other devices continue to work fine.

Anyone seen anything like this or have any suggestions. I'm not a regular ipad user so
there may be some sort of safari reset that I could try..

Thanks in advance