floatpanels for comboboxes are offset left of related combobox

[ngiesen]

When a combobox (such as 'Styles','Format','Font' or 'FontSize') is positioned with its right edge against the window edge, its corresponding floating panel will have its right edge tucked to the left of the combobox; it should tuck to the right.

I hope the following pieces of ascii-art will illustrate the issue:

Current behaviour:

                    |window edge
                    |
            [Styles]|
[opt1      ]        |
[opt2      ]        |
[opt3      ]        |

Preferred behaviour:

                    |window edge
                    |
            [Styles]|
        [opt1      ]|
        [opt2      ]|
        [opt3      ]|

[Jakub Ś]

No longer reproducible in latest trunk.