Extra vertical space between two paragraphs added by figure

Question

Compiling the following code
documentclass{article}

usepackage{lipsum}

begin{document}
lipsum[1] aaa aaa aaa aaa aaa aaa aaa aaa aaa  aaa aaa aaa aaa aaa aaa aaa
aaaa

lipsum[1]
end{document}

returns two paragraphs as expected.

But if a figure is placed after the first paragraph,
documentclass{article}

usepackage{lipsum}

begin{document}
lipsum[1] aaa aaa aaa aaa aaa aaa aaa aaa aaa  aaa aaa aaa aaa aaa aaa aaa
aaaa
begin{figure}
  caption{A}
end{figure}

lipsum[1]
end{document}

LaTeX inserts a vertical space between the two paragraphs.

If a %-character is added at the end of the first paragraph or a blank line is added before the figure, the vertical space disappears and the ouptut matches the first one. This only seems to happen if the first paragraph ends in a full line.
Why does this happen and should therefore every float be preceded with a blank line?

David Carlisle · Accepted Answer

It is not really related to figure you see the same from here
documentclass{article}

usepackage{lipsum}

begin{document}
lipsum[1] aaa aaa aaa aaa aaa aaa aaa aaa aaa  aaa aaa aaa aaa aaa aaa aaa
aaaa mbox{}

lipsum[1]
end{document}

It is not actually vertical space, it is an extra all white line of the first paragraph (you can most easily see the difference if a page break occurs as this is a paragraph line it is not dropped at the top of a page, as vertical space would be.
The primitive end-of-paragraph process does one unskip to remove a space at the end of a paragraph which usually comes from the end of line, but you have space-emptybox-space so you end up with space-emptybox at the end, then you were unlucky and got a linebreak at the space, so leaving the box on its own.
Actually with figure you get a vadjust node not a box, but it has the same effect.

Extra vertical space between two paragraphs added by figure

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