If a PDF is attached to a bibliography entry item by dragging it to the drawer of the entry editing window, when it is first created, the thumbnail PDF of the preview will appear as a blank white page, regardless of what the first page of the PDF is. Once the entry is saved, BibDesk restarted, and the entry selected in the main window of the app, the thumbnail preview will then be displayed correctly.
This bug does not happen if (i) the entry is created, (ii) the editing window for the entry is closed, (iii) the new entry selected in the main App window, and (iv) the PDF is then attached by dragging it to the drawer.
I cannot reproduce this problem, for me it just displays the first page as expected. Your description also does not make much sense, as the drawing of the icon is exactly the same no matter whether the detail window is open or not (it does not happen in the window). That must be a spurious coincidence. I don't think we can do anything about this when it happens, when the PDF library doesn't draw it, I don't see how we can force it to do so (this is Apple's black box drawing it).
BTW, when this happens you can also try to redraw the icon using the Reload item in the contextual menu.
I cannot reproduce this problem, for me it just displays the first page as expected. Your description also does not make much sense, as the drawing of the icon is exactly the same no matter whether the detail window is open or not (it does not happen in the window). That must be a spurious coincidence. I don't think we can do anything about this when it happens, when the PDF library doesn't draw it, I don't see how we can force it to do so (this is Apple's black box drawing it).
BTW, when this happens you can also try to redraw the icon using the Reload item in the contextual menu.
I also have the same issue. Sometimes the first page of a pdf is displayed, sometimes not. Interestingly, only the first page seems to be affected. The following pages are displayed properly.
I finally was able to track this down. It will now be fixed in the next release.