Thanks - but this is a Pixinsight window, not a windows one!
Try it - maximise the real time preview window and see if you can shut it - when maximised, the normal controls do not appear (iconise, shade, maximise, close). This is actually true for any image window if you maximise it - (instead these controls appear in the top right of the screen) and there are also controls on the bottom left (e.g. zoom to optimal fit), or cascade windows which restore the geometry.
None of these methods works for the real time preview window.
Colin