Make sure you pay special attention to the panel icon filename. If you don’t have the “@1x” at the end of the name (before the extension), the panel icon will look fine on Mac but will not show up on Windows.
And the size suffixes (“@1x” etc.) go only on the filenames, not in the manifest file. So this icon:
panelIcon@1x.png
Is referred to in the manifest simply as “panelIcon.png”