fix(ui): merge article blocks to fix scroll jank

Link-heavy articles split every line into its own list item, so a
fast fling composed a dozen items in one frame and froze. Merge small
blocks into multi-paragraph items: indent via em space and a uniform
line pitch via LineHeightStyle(Proportional, Trim.Both).

app/src/main/java/com/example/research/ui/article/ArticleScreen.kt

@@ -18,6 +18,9 @@ import androidx.compose.ui.res.stringResource
 import androidx.compose.ui.semantics.semantics
 import androidx.compose.ui.semantics.testTagsAsResourceId
 import androidx.compose.ui.unit.dp
+import androidx.compose.ui.text.AnnotatedString
+import androidx.compose.ui.text.TextStyle
+import androidx.compose.ui.text.style.LineHeightStyle
 import androidx.compose.ui.text.style.TextOverflow
 import androidx.compose.ui.unit.IntSize
 import androidx.lifecycle.compose.collectAsStateWithLifecycle
@@ -48,6 +51,54 @@ fun ArticleRoute(
     )
 }
 
+private const val MERGED_BLOCK_MAX_CHARS = 1500
+private const val MERGED_BLOCK_MAX_ITEMS = 12
+
+private fun mergeDslBlocks(blocks: List<DslBlock>): List<DslBlock> {
+    if (blocks.size <= MERGED_BLOCK_MAX_ITEMS) return blocks
+    val merged = ArrayList<DslBlock>(blocks.size / 4)
+    var i = 0
+    while (i < blocks.size) {
+        val builder = AnnotatedString.Builder()
+        val refs = ArrayList<DslAnnotatedParser.LazyRef>()
+        var j = i
+        while (j < blocks.size) {
+            val block = blocks[j]
+            repeat(block.indent) { builder.append(' ') }
+            val paraStart = builder.length
+            builder.append(block.text)
+            block.lazyRefs.forEach { ref ->
+                refs.add(
+                    ref.copy(
+                        startIndex = ref.startIndex + paraStart,
+                        endIndex = ref.endIndex + paraStart
+                    )
+                )
+            }
+            val more = j + 1 < blocks.size &&
+                (j + 1 - i) < MERGED_BLOCK_MAX_ITEMS &&
+                builder.length < MERGED_BLOCK_MAX_CHARS
+            if (more) {
+                builder.append('\n')
+            }
+            j++
+            if (!more) {
+                break
+            }
+        }
+        merged.add(
+            DslBlock(
+                text = builder.toAnnotatedString(),
+                indent = 0,
+                lazyRefs = refs,
+                continuesPrevious = blocks[i].continuesPrevious
+            )
+        )
+        i = j
+    }
+    return merged
+}
+
 @Composable
 fun ArticleContent(
     articleState: ArticleState?,
@@ -63,7 +114,11 @@ fun ArticleContent(
         is ArticleState.Success -> {
             val articleSaveKey = "${articleState.dictionaryPath}:${articleState.title}"
             val baseTextStyle = MaterialTheme.typography.bodyLarge.copy(
-                color = MaterialTheme.colorScheme.onSurface
+                color = MaterialTheme.colorScheme.onSurface,
+                lineHeightStyle = LineHeightStyle(
+                    alignment = LineHeightStyle.Alignment.Proportional,
+                    trim = LineHeightStyle.Trim.Both,
+                ),
             )
             val indentStep = 16.dp
 
@@ -123,6 +178,12 @@ fun ArticleContent(
             val isHeavyArticle =
                 articleState.dslContent.size > HEAVY_ARTICLE_BLOCK_COUNT
 
+            val displayBlocks = remember(
+                articleState.dslContent,
+                baseTextStyle
+            ) {
+                mergeDslBlocks(articleState.dslContent)
+            }
             val articleList: @Composable (Modifier) -> Unit = { listModifier ->
                 LazyColumn(
                     state = listState,
@@ -134,39 +195,30 @@ fun ArticleContent(
                         }
                 ) {
                     itemsIndexed(
-                        items = articleState.dslContent,
+                        items = displayBlocks,
                         key = { index, _ ->
-                            // Use stable key combining article save key and index
                             "$articleSaveKey:$index"
                         },
                         contentType = { _, _ -> "article_block" }
                     ) { index, block ->
-                        val followedByContinuation =
-                            articleState.dslContent.getOrNull(index + 1)?.continuesPrevious == true
-                        val textContent = @Composable {
-                            Box(
+                        Box(
+                            modifier = Modifier
+                                .padding(start = Spacing.contentStart + indentStep * block.indent)
+                        ) {
+                            ArticleLinkText(
+                                block = block,
+                                linksEnabled = true,
+                                preferDirectHitTesting = isHeavyArticle,
+                                style = baseTextStyle,
+                                onRefClick = refClickHandler,
+                                onTextLayout = {},
                                 modifier = Modifier
-                                    .padding(
-                                        start = Spacing.contentStart + indentStep * block.indent
-                                    )
-                                    .padding(bottom = if (followedByContinuation) 0.dp else 8.dp)
-                            ) {
-                                ArticleLinkText(
-                                    block = block,
-                                    linksEnabled = true,
-                                    preferDirectHitTesting = isHeavyArticle,
-                                    style = baseTextStyle,
-                                    onRefClick = refClickHandler,
-                                    onTextLayout = {},
-                                    modifier = Modifier
-                                        .testTag("article_block_$index")
-                                        .semantics {
-                                            testTagsAsResourceId = true
-                                        }
-                                )
-                            }
+                                    .testTag("article_block_$index")
+                                    .semantics {
+                                        testTagsAsResourceId = true
+                                    }
+                            )
                         }
-                        textContent()
                     }
                 }
             }