merge-paragraphs.html

<div contenteditable="true">
    <div>abc</div><div>def</div>

    <br>
    <br>

    <div>abc</div><div>def<span contenteditable="false">noteditable</span></div>

    <br>
    <br>

    <div>abc</div>def

    <br>
    <br>

    <div>abc</div>def<span contenteditable="false">noteditable</span>

    <br>
    <br>

    abc<div>def</div>

    <br>
    <br>

    abc<div>def<span contenteditable="false">noteditable</span></div>

    <br>
    <br>

    <div>abc<span contenteditable="false">noteditable</span></div><div>def</div>

    <br>
    <br>

    <div>abc<span contenteditable="false">noteditable</span></div>def

    <br>
    <br>

    <div><span contenteditable="true">noteditable</span>abc</div><div>def</div>

    <br>
    <br>

    <span contenteditable="true">noteditable</span>abc<div>def</div>

    <br>
    <br>

    <div>abc</div><div><span contenteditable="false">noteditable</span>def</div>

    <br>
    <br>

    <div>abc<br>def</div>

    <br>
    <br>

    abc<br>def

    <br>
    <br>

    abc<div><br></div>def

    <br>
    <br>

    <div><br></div>abc

    <br>
    <br>

    abc<span style="white-space: pre;">&#10;</span>def

    <br>
    <br>

    abc<div><span style="white-space: pre;">&#10;</span></div>def

    <br>
    <br>

    <div><span style="white-space: pre;">&#10;</span></div>abc
</div>