BUG: Centre pane window display shifts when selecting thumbnail

[rkferguson]

Reported by Iain Porter - Iain says new with 5.1.1 - using Window 7

When you have a number of datasets selected and display them all in the centre pane - with lots of rows of thumbnails showing in centre pane - scroll to bottom and select a thumbnail.
Centre pane display jumps to top right corner, so you cannot see the selected thumbnail - have to scroll horizontally and vertically to locate it again. Screenshots 1 and 2.

Tested on Windows 8 with 5.1.1 - present and 5.1.0 - present.
Tested with Mac 5.1.1 and 5.1.0 - not present.

Noted by Gus:

• in Windows 7 horizontal scrollbar appears unnecessarily because centre pane display is sizing itself to be unnecessarily large when displaying multiple datasets - this does not appear to affect Windows 8.
• if centre pane is made wider and then narrower again, it does not re-arrange thumbnails and return to narrower display - remains at widest used - this is present in Windows and Mac 5.1.0 and 5.1.1) - screenshots 3 and 4
• in 5.1.0 the centre pane horizontal scroll bar is always present as well, although the display is only a small amount larger than the pane compared with the large amount in 5.1.1. - screenshot 3