Fix display of horizontal lines in dashboard text cards (#20177)

frontend/src/metabase/visualizations/visualizations/Text.css

@@ -155,6 +155,10 @@
   height: auto;
 }
 
+:local .text-card-markdown hr {
+  margin: 0;
+}
+
 :local .single-row {
   margin: 0;
 }

src/metabase/pulse/markdown.clj

@@ -228,7 +228,10 @@
       :hard-line-break
       "\n"
 
-      (:heading)
+      :horizontal-line
+      "\n───────────────────\n"
+
+      :heading
       (str "*" joined-content "*\n")
 
       :bold

test/metabase/pulse/markdown_test.clj

@@ -40,6 +40,10 @@
     (is (= "foo\nbar" (mrkdwn "foo  \nbar")))
     (is (= "foo\nbar" (mrkdwn "foo\\\nbar"))))
 
+  (testing "Horizontal lines are created using box drawing characters"
+    (is (= "───────────────────" (mrkdwn "----")))
+    (is (= "text\n\n───────────────────\ntext" (mrkdwn "text\n\n----\ntext"))))
+
   (testing "Code blocks are preserved"
     (is (= "`code`"                (mrkdwn "`code`")))
     (is (= "```\ncode\nblock```"   (mrkdwn "    code\n    block")))