fix: render markdown content on re-attachment (#7808) (#7810)

* Initial plan

* Fix re-attaching Markdown component content disappearing issue



* run formatter

---------

Co-authored-by: Copilot <[email protected]>
Co-authored-by: sissbruecker <[email protected]>
Co-authored-by: Sascha Ißbrücker <[email protected]>

Author: 3 people

vaadin-markdown-flow-parent/vaadin-markdown-flow/src/main/java/com/vaadin/flow/component/markdown/Markdown.java

@@ -17,6 +17,7 @@
 
 import java.util.Objects;
 
+import com.vaadin.flow.component.AttachEvent;
 import com.vaadin.flow.component.Component;
 import com.vaadin.flow.component.HasSize;
 import com.vaadin.flow.component.Tag;
@@ -89,6 +90,15 @@ public String getContent() {
         return serverContent;
     }
 
+    @Override
+    protected void onAttach(AttachEvent attachEvent) {
+        super.onAttach(attachEvent);
+        if (serverContent != null) {
+            clientContent = null;
+            scheduleContentUpdate();
+        }
+    }
+
     private void scheduleContentUpdate() {
         getElement().getNode()
                 .runWhenAttached(ui -> ui.beforeClientResponse(this, ctx -> {

vaadin-markdown-flow-parent/vaadin-markdown-flow/src/test/java/com/vaadin/flow/component/markdown/tests/MarkdownTest.java

@@ -138,6 +138,21 @@ public void testDetach_setContent_attach() {
         Assert.assertEquals("**Hello** _World_", markdown.getContent());
     }
 
+    @Test
+    public void testReAttachment_contentPreserved() {
+        // Set content while attached
+        markdown.setContent("**Hello** _World_");
+        assertUpdateMarkdownCall(markdown, "**Hello** _World_", false);
+
+        // Detach the component
+        markdown.removeFromParent();
+
+        // Re-attach the component - content should be sent to client again
+        ui.add(markdown);
+        assertUpdateMarkdownCall(markdown, "**Hello** _World_", false);
+        Assert.assertEquals("**Hello** _World_", markdown.getContent());
+    }
+
     private void assertUpdateMarkdownCall(Component component, String content,
             boolean isAppend) {
         var pendingJavaScriptInvocations = getPendingJavaScriptInvocations();