illustration of a peculiarity of TeX paragraph building

figure.tex

\documentclass{article}

\usepackage{mwe}
\begin{document}

% or anything filling the line with less than inter-word space shrinkability
\noindent\rule{\linewidth}{1pt} % <-- we deliberately leave a space token here
\begin{figure}[htbp]
  \centering
  % only by way of example, close to real life examples
  \includegraphics[height=1cm]{example-image-a}
  \caption{Hello, is there extra space above figure env?}
\end{figure}

\noindent\rule{\linewidth}{1pt}

\begin{figure}[htbp]
  \centering
  \includegraphics[height=1cm]{example-image-a}
  \caption{This one doesn't have any...}
\end{figure}
\end{document}

\hskip@savsk % ADDED
\ifdim\lastskip=\z@

for example can't be right as written as the test will never be true (that's probably fixable but this is a very delicate area, and early on in 2e we tried several variants all broke something)

@davidcarlisle Yes of course, the \ifdim will never be true, I just dropped the extra line in for checking it worked in my use case, I should I have put it after but didn't like the \nobreak, and didn't think twice

about figure at end of paragraph, the \ignorespaces it still there. It does make the issue go away in my testing. the ignorespaces is not relevant here. It works without it. (see next comment)

for example with

AAAAAAAA  AAAAAAAA  AAAAAAAA  AAAAAAAA  AAAAAAAA IIII.
\begin{figure}[htbp]
  \centering
  x
  \caption{Hello, is there extra space above figure env?}
\end{figure} {} {}

and the modified sphack, there is no issue.