LaTeX reader: handle `%` comment right after command.

For example

    \emph%
    {hi}

3 files changed, 55 insertions, 1 deletions

diff --git a/src/Text/Pandoc/Readers/LaTeX.hs b/src/Text/Pandoc/Readers/LaTeX.hs
index a982029af..9bac3d3a7 100644
--- a/src/Text/Pandoc/Readers/LaTeX.hs
+++ b/src/Text/Pandoc/Readers/LaTeX.hs
@@ -1132,7 +1132,7 @@ inlineCommand' = try $ do
   lookupListDefault raw names inlineCommands
 
 tok :: PandocMonad m => LP m Inlines
-tok = grouped inline <|> inlineCommand' <|> singleChar'
+tok = try $ spaces >> grouped inline <|> inlineCommand' <|> singleChar'
   where singleChar' = do
           Tok _ _ t <- singleChar
           return (str (T.unpack t))
diff --git a/test/command/4016.md b/test/command/4016.md
new file mode 100644
index 000000000..69ad1c911
--- /dev/null
+++ b/test/command/4016.md
@@ -0,0 +1,47 @@
+```
+pandoc -t beamer
+# Level 2 blocks
+
+<div class="columns">
+<div class="column" width="40%">
+## Block one
+- Item
+</div>
+<div class="column" width="60%">
+## Block two
+- Item
+</div>
+</div>
+^D
+\begin{frame}{%
+\protect\hypertarget{level-2-blocks}{%
+Level 2 blocks}}
+
+\begin{columns}[T]
+\begin{column}{0.40\textwidth}
+\begin{block}{Block one}
+
+\begin{itemize}
+\tightlist
+\item
+  Item
+\end{itemize}
+
+\end{block}
+\end{column}
+
+\begin{column}{0.60\textwidth}
+\begin{block}{Block two}
+
+\begin{itemize}
+\tightlist
+\item
+  Item
+\end{itemize}
+
+\end{block}
+\end{column}
+\end{columns}
+
+\end{frame}
+```
diff --git a/test/command/latex-command-comment.md b/test/command/latex-command-comment.md
new file mode 100644
index 000000000..640277f15
--- /dev/null
+++ b/test/command/latex-command-comment.md
@@ -0,0 +1,7 @@
+```
+pandoc -f latex -t native
+\emph%
+{hi}
+^D
+[Para [Emph [Str "hi"]]]
+```