how to start a new line after inserting a figure without indentation at the beginning of this new line?

This looks like a bug in the float package, here is the start of a fix, I show the image included with and without a blank line after the figure

\documentclass[a4paper,12pt]{article}

\usepackage[english]{babel}
\usepackage{graphicx}
\usepackage{float}

\makeatletter

\renewcommand\float@endH{\@endfloatbox\vskip\intextsep
  \if@flstyle\setbox\@currbox\float@makebox\columnwidth\fi
  \box\@currbox\vskip\intextsep\relax\@doendpe}

\makeatother
\begin{document}
\section{Paragraph}

xx

Here are some words in the last paragraph. Now a figure is inserted below.
\begin{figure}[H]
\caption{A picture of a gull.}
\centering
\includegraphics[width=0.5\textwidth]{example-image}
\end{figure}
Now I want to start a new line after this figure. But there is always indentation at the begining. How to delete this indentation?

Here are some words in the last paragraph. Now a figure is inserted below.
\begin{figure}[H]
\caption{A picture of a gull.}
\centering
\includegraphics[width=0.5\textwidth]{example-image}
\end{figure}

Now I want to start a new line after this figure. But there is always indentation at the begining. How to delete this indentation?






\end{document}

You need to include \noindent at the beginning of the first line.

\documentclass[a4paper,12pt,demo]{article}
\usepackage[english]{babel}
\usepackage{graphicx}
\usepackage{float}

\begin{document}
\section{Paragraph}
Here are some words in the last paragraph. Now a figure is inserted below.
\begin{figure}[H]
\caption{A picture of a gull.}
\centering
\includegraphics[width=0.5\textwidth]{Penguins.jpg}
\end{figure}
\noindent Now I want to start a new line after this figure. But there is always indentation at the begining. How to delete this indentation?
\end{document}

That leeds to: