Fix regression in handling of columns in beamer slides.

Columns in title slides were causing problems with
slide division.  Closes #6033.

src/Text/Pandoc/Writers/LaTeX.hs

@@ -460,13 +460,14 @@ elementToBeamer slideLevel (Div (ident,"section":dclasses,dkvs)
   | lvl >  slideLevel
     = return $ Div (ident,"block":dclasses,dkvs) xs
   | lvl <  slideLevel
-    = do let isDiv (Div{})      = True
-             isDiv _            = False
-         let (titleBs, slideBs) = break isDiv ys
+    = do let isSlide (Div (_,"slide":_,_) _)   = True
+             isSlide (Div (_,"section":_,_) _) = True
+             isSlide _                         = False
+         let (titleBs, slideBs) = break isSlide ys
          return $
            if null titleBs
-              then Div (ident,dclasses,dkvs) xs
-              else Div (ident,dclasses,dkvs)
+              then Div (ident,"section":dclasses,dkvs) xs
+              else Div (ident,"section":dclasses,dkvs)
                     (h : Div ("","slide":dclasses,dkvs) (h:titleBs) : slideBs)
   | otherwise
     = return $ Div (ident,"slide":dclasses,dkvs) xs

test/command/6033.md

@@ -0,0 +1,91 @@
+```
+% pandoc -t beamer --slide-level=4
+# One
+
+## One-A
+
+:::: columns
+
+::: column
+
+- One column.
+
+:::
+
+::: column
+
+- Another column.
+
+:::
+
+::::
+
+- Some more text.
+
+## One-B
+
+- Text.
+
+# Two
+
+## Two-A
+
+- More text.
+^D
+\hypertarget{one}{%
+\section{One}\label{one}}
+
+\hypertarget{one-a}{%
+\subsection{One-A}\label{one-a}}
+
+\begin{frame}{One-A}
+\begin{columns}[T]
+\begin{column}{0.48\textwidth}
+\begin{itemize}
+\tightlist
+\item
+  One column.
+\end{itemize}
+\end{column}
+
+\begin{column}{0.48\textwidth}
+\begin{itemize}
+\tightlist
+\item
+  Another column.
+\end{itemize}
+\end{column}
+\end{columns}
+
+\begin{itemize}
+\tightlist
+\item
+  Some more text.
+\end{itemize}
+\end{frame}
+
+\hypertarget{one-b}{%
+\subsection{One-B}\label{one-b}}
+
+\begin{frame}{One-B}
+\begin{itemize}
+\tightlist
+\item
+  Text.
+\end{itemize}
+\end{frame}
+
+\hypertarget{two}{%
+\section{Two}\label{two}}
+
+\hypertarget{two-a}{%
+\subsection{Two-A}\label{two-a}}
+
+\begin{frame}{Two-A}
+\begin{itemize}
+\tightlist
+\item
+  More text.
+\end{itemize}
+\end{frame}
+```