Add system tests for markdown paste in Lexxy editor

Test that pasting markdown with block separators produces extra spacing
between blocks, and that single line breaks are preserved.

test/system/smoke_test.rb

@@ -76,6 +76,38 @@ class SmokeTest < ApplicationSystemTestCase
     assert_no_selector "div##{dom_id(notification)}"
   end
 
+  test "markdown paste adds block spacing" do
+    sign_in_as(users(:david))
+
+    visit card_url(cards(:layout))
+    find("lexxy-editor").click
+    paste_markdown("Hello\n\nWorld")
+
+    within("lexxy-editor") do
+      assert_selector "p", text: "Hello"
+      assert_selector "p br", visible: :all
+      assert_selector "p", text: "World"
+    end
+  end
+
+  test "markdown paste preserves line breaks" do
+    sign_in_as(users(:david))
+
+    visit card_url(cards(:layout))
+    find("lexxy-editor").click
+    paste_markdown("Hello\nWorld")
+
+    inner_html = find("lexxy-editor p", text: "Hello").native.property("innerHTML")
+    children = Nokogiri::HTML5.fragment(inner_html).children
+    assert_pattern do
+      children => [
+        { name: "span", inner_html: "Hello" },
+        { name: "br" },
+        { name: "span", inner_html: "World" }
+      ]
+    end
+  end
+
   test "dragging card to a new column" do
     sign_in_as(users(:david))
 
@@ -105,4 +137,12 @@ class SmokeTest < ApplicationSystemTestCase
       editor_element.set with
       page.execute_script("arguments[0].value = '#{with}'", editor_element)
     end
+
+    def paste_markdown(markdown)
+      page.execute_script(<<~JS, markdown)
+        const dt = new DataTransfer();
+        dt.setData("text/plain", arguments[0]);
+        document.activeElement.dispatchEvent(new ClipboardEvent("paste", { clipboardData: dt, bubbles: true }));
+      JS
+    end
 end