I think I did not explain thoroughly:
- right-click on single file and select Save As (including following through with the actual Save As, including inputting desired filename, file-location (I chose Desktop, but I don't think this matters), and hitting Save button — the file saves and dialog box closes
- close Sigil window and everything looks fine (i.e., all windows are gone and your newly saved file is in the location you chose)
- click on Sigil icon in Dock or open new epub (with right-click or double-click)
- Save As dialog box re-appears (even though the Save As has already been completed in the first step above) and the only way to get rid of it is to quit Sigil