I was playing around with the preheader text that displays under the subject line and becomes invisible/disappears on the actual body of the email.

So something like this that's inserted immediately after the <body> tag:
<span style="display:none !important; color:transparent; font-size:0px; height:0; widows:0; visibility:hidden;">Preheader Text</span>

I noticed that if the text in that <span> is very short, the preheader does display that text, but afterwards it also displays the first content text in the body of the email. (I see this on mobile email clients especially) Are there any methods to make the preheader display only what is enclosed in the <span> tag?

Also, is there a way to remove the preheader text entirely so that what displays in the preheader is just blank? (I'm mostly interested in this for testing purposes.)

I've tried &nbsp; and putting emptly <div>, <span>, and <p> after it. None of that seems to work.