test: add regression tests for copy line-break preservation (#626)

Issue #626 reported that copying styled text from the editor and
pasting into a plain-text target (e.g. the Android notes app)
collapsed everything onto a single line. The issue was filed on
2025-10-21, before rich clipboard support landed in 3770676
(2026-04-11, PR #642).

Before #642, the library relied on Compose's default copy, which
reads from the transformed annotated string — and since the
annotated string replaces '\n' with ' ' for rendering, the plain
text payload lost paragraph breaks. #642 overrode setClipEntry on
all platforms to build the payload from toText(copySelection) and
toHtml(copySelection), both of which preserve '\n' and block tags.

These tests lock in that behavior across seven scenarios:
- copy across <p> paragraphs (plain and styled)
- copy across <br> paragraphs (isFromLineBreak = true)
- copy with mixed inline styles across <br> boundaries
- mid-range copy that crosses a paragraph boundary
- copy across an ordered list
- toHtml(copySelection) preserves block-level tags

Marking #626 as already fixed by #642.

Author: MohamedRejeb

richeditor-compose/src/commonTest/kotlin/com/mohamedrejeb/richeditor/model/RichTextStateCopyLineBreaksTest.kt

@@ -0,0 +1,126 @@
+package com.mohamedrejeb.richeditor.model
+
+import androidx.compose.ui.text.TextRange
+import com.mohamedrejeb.richeditor.annotation.ExperimentalRichTextApi
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertTrue
+
+/**
+ * Regression tests for issue #626 — copied plain text loses line
+ * breaks when the selection spans multiple paragraphs or line-break
+ * ([isFromLineBreak]) paragraphs produced from HTML `<br>`.
+ *
+ * The clipboard plain-text payload is produced by
+ * [RichTextState.toText] with the copy selection. When the text
+ * lacks `\n` between paragraphs, pasting into a plain-text target
+ * (e.g. a notes app) collapses everything onto a single line.
+ */
+@OptIn(ExperimentalRichTextApi::class)
+class RichTextStateCopyLineBreaksTest {
+
+    @Test
+    fun testCopyAcrossParagraphsPreservesLineBreaks() {
+        val state = RichTextState()
+        state.setHtml("<p>First paragraph</p><p>Second paragraph</p><p>Third paragraph</p>")
+        val text = state.textFieldValue.text
+
+        val copied = state.toText(TextRange(0, text.length))
+
+        assertEquals(2, copied.count { it == '\n' }, "Three paragraphs should be separated by two newlines: <$copied>")
+        assertTrue(copied.contains("First"))
+        assertTrue(copied.contains("Second"))
+        assertTrue(copied.contains("Third"))
+    }
+
+    @Test
+    fun testCopyAcrossStyledParagraphsPreservesLineBreaks() {
+        val state = RichTextState()
+        state.setHtml(
+            "<p><b>First paragraph</b></p>" +
+                "<p><i>Second paragraph</i></p>" +
+                "<p><u>Third paragraph</u></p>"
+        )
+        val text = state.textFieldValue.text
+
+        val copied = state.toText(TextRange(0, text.length))
+
+        assertEquals(
+            "First paragraph\nSecond paragraph\nThird paragraph",
+            copied,
+        )
+    }
+
+    @Test
+    fun testCopyAcrossBrLineBreaksPreservesNewlines() {
+        // <br> inside a <p> produces sibling paragraphs with isFromLineBreak = true
+        val state = RichTextState()
+        state.setHtml("<p>Line one<br>Line two<br>Line three</p>")
+        val text = state.textFieldValue.text
+
+        val copied = state.toText(TextRange(0, text.length))
+
+        assertEquals(
+            "Line one\nLine two\nLine three",
+            copied,
+        )
+    }
+
+    @Test
+    fun testCopyMidRangeAcrossParagraphsPreservesLineBreak() {
+        val state = RichTextState()
+        state.setHtml("<p>First paragraph</p><p>Second paragraph</p>")
+        val text = state.textFieldValue.text
+
+        // Select "paragraph\nSecond"
+        val start = text.indexOf("paragraph")
+        val end = text.indexOf("Second") + "Second".length
+        val copied = state.toText(TextRange(start, end))
+
+        assertEquals("paragraph\nSecond", copied)
+    }
+
+    @Test
+    fun testCopyAcrossOrderedListPreservesLineBreaks() {
+        val state = RichTextState()
+        state.setHtml("<ol><li>First</li><li>Second</li><li>Third</li></ol>")
+        val text = state.textFieldValue.text
+
+        val copied = state.toText(TextRange(0, text.length))
+
+        assertEquals(2, copied.count { it == '\n' }, "List items should be separated by newlines: <$copied>")
+    }
+
+    @Test
+    fun testCopyAcrossBrParagraphsWithStylesPreservesLineBreaks() {
+        val state = RichTextState()
+        state.setHtml(
+            "<p><b>Bold line</b><br><i>Italic line</i><br><u>Underline line</u></p>"
+        )
+        val text = state.textFieldValue.text
+
+        val copied = state.toText(TextRange(0, text.length))
+
+        assertEquals(
+            "Bold line\nItalic line\nUnderline line",
+            copied,
+        )
+    }
+
+    @Test
+    fun testCopyAcrossParagraphsProducesHtmlWithBlockBreaks() {
+        // Clipboard consumers that read the HTML payload (e.g. Android's
+        // Html.fromHtml in receiving apps) rely on block-level tags to insert
+        // line breaks. Ensure the HTML preserves paragraph structure.
+        val state = RichTextState()
+        state.setHtml("<p>First paragraph</p><p>Second paragraph</p>")
+        val text = state.textFieldValue.text
+
+        val html = state.toHtml(TextRange(0, text.length))
+
+        assertTrue(
+            html.contains("<p") || html.contains("<br"),
+            "Copied HTML should contain block or line-break tags to preserve paragraphs: <$html>",
+        )
+    }
+}