Fix toPlainText where <ol start='n'> tags appear (#5044)

Author: jmartinesp

libraries/matrixui/src/main/kotlin/io/element/android/libraries/matrix/ui/messages/ToPlainText.kt

@@ -57,10 +57,16 @@ private class PlainTextNodeVisitor : NodeVisitor {
         if (node is TextNode && node.text().isNotBlank()) {
             builder.append(node.text())
         } else if (node is Element && node.tagName() == "li") {
-            val index = node.elementSiblingIndex()
+            val index = node.elementSiblingIndex() + 1
             val isOrdered = node.parent()?.nodeName()?.lowercase() == "ol"
             if (isOrdered) {
-                builder.append("${index + 1}. ")
+                val startIndex = node.parent()?.attr("start")?.toIntOrNull()
+                val actualIndex = if (startIndex != null) {
+                    startIndex + index - 1
+                } else {
+                    index
+                }
+                builder.append("$actualIndex. ")
             } else {
                 builder.append("• ")
             }

libraries/matrixui/src/test/kotlin/io/element/android/libraries/matrix/ui/messages/ToPlainTextTest.kt

@@ -96,6 +96,29 @@ class ToPlainTextTest {
         )
     }
 
+    @Test
+    fun `TextMessageType toPlainText - respects the ol start attr if present`() {
+        val messageType = TextMessageType(
+            body = "1. First item\n2. Second item\n",
+            formatted = FormattedBody(
+                format = MessageFormat.HTML,
+                body = """
+                    <ol start='11'>
+                        <li>First item.</li>
+                        <li>Second item.</li>  
+                    </ol>
+                    <br />
+                """.trimIndent()
+            )
+        )
+        assertThat(messageType.toPlainText(permalinkParser = FakePermalinkParser())).isEqualTo(
+            """
+            11. First item.
+            12. Second item.
+            """.trimIndent()
+        )
+    }
+
     @Test
     fun `TextMessageType toPlainText - returns the markdown body if the formatted one cannot be parsed`() {
         val messageType = TextMessageType(