From d17652218a99be822ea7f384b9d90b6ec1d47476 Mon Sep 17 00:00:00 2001
From: Keisuke KATO <kasecato@gmail.com>
Date: Sat, 17 Feb 2018 05:59:33 +0900
Subject: [PATCH] Fixed an error when previewing the first line with Markdown
 extension (#43835)

---
 extensions/markdown/media/main.js | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/extensions/markdown/media/main.js b/extensions/markdown/media/main.js
index 53ac317d851..a3128f30609 100644
--- a/extensions/markdown/media/main.js
+++ b/extensions/markdown/media/main.js
@@ -155,9 +155,10 @@
 		if (previous) {
 			if (next) {
 				const betweenProgress = (offset - window.scrollY - previous.element.getBoundingClientRect().top) / (next.element.getBoundingClientRect().top - previous.element.getBoundingClientRect().top);
-				return previous.line + betweenProgress * (next.line - previous.line);
+				const line = previous.line + betweenProgress * (next.line - previous.line);
+				return (0 <= line) ? line : 0;
 			} else {
-				return previous.line;
+				return (0 <= previous.line) ? previous.line : 0;
 			}
 		}
 		return null;