Handle different behavior from mochihtml vs Lexbor when adding butto styles to Markdown campaigns

Author: wmnnd

lib/keila/templates/html.ex

@@ -163,35 +163,19 @@ defmodule Keila.Templates.Html do
   Apply markup transforms for improved email client compatibility.
 
   ## Transforms:
-  - `h4 a` -> `div.keila-button a`
+  - `h4 a` -> `div.keila-button a` (this is implemented twice because mochihtml strips the leading "\n" while Lexbor retains it)
   """
   @spec apply_email_markup(t()) :: t()
   def apply_email_markup(html) do
     Floki.traverse_and_update(html, &do_apply_email_markup/1)
   end
 
   defp do_apply_email_markup({"h4", _, [{"a", a_attrs, a_children}]}) do
-    a_attrs =
-      case Enum.find_index(a_attrs, &(elem(&1, 0) == "class")) do
-        nil ->
-          [{"class", "button-a"} | a_attrs]
-
-        i ->
-          List.update_at(a_attrs, i, fn {"class", classes} ->
-            {"class", classes <> " button-a"}
-          end)
-      end
+    build_button_markup(a_attrs, a_children)
+  end
 
-    {"table", [{"style", "width: 100%"}],
-     [
-       {"tr", [{"class", "block--button"}],
-        [
-          {"td", [{"class", "button-td"}],
-           [
-             {"a", a_attrs, a_children}
-           ]}
-        ]}
-     ]}
+  defp do_apply_email_markup({"h4", _, ["\n", {"a", a_attrs, a_children}]}) do
+    build_button_markup(a_attrs, a_children)
   end
 
   defp do_apply_email_markup({"blockquote", _, content}) do
@@ -248,4 +232,28 @@ defmodule Keila.Templates.Html do
   end
 
   defp do_apply_email_markup(other), do: other
+
+  defp build_button_markup(a_attrs, a_children) do
+    a_attrs =
+      case Enum.find_index(a_attrs, &(elem(&1, 0) == "class")) do
+        nil ->
+          [{"class", "button-a"} | a_attrs]
+
+        i ->
+          List.update_at(a_attrs, i, fn {"class", classes} ->
+            {"class", classes <> " button-a"}
+          end)
+      end
+
+    {"table", [{"style", "width: 100%"}],
+     [
+       {"tr", [{"class", "block--button"}],
+        [
+          {"td", [{"class", "button-td"}],
+           [
+             {"a", a_attrs, a_children}
+           ]}
+        ]}
+     ]}
+  end
 end