populate children of unit collapsed drop location

Just a patch for avoiding removal of existing children while drag & drop unit into a collapsed section.
This doesn't work because the method is called in more than one place. For instance, if you drag an expanded subsection, the drag and drop code first collapses the subsection (during the drag), then calls this method to re-expand it on drop. This trigger of ui-toggle-expansion causes the dragged element to collapse again.