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
paragraph spacing via baseline shift.

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

@@ -12,13 +12,20 @@ import androidx.compose.runtime.saveable.rememberSaveable
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.layout.onSizeChanged
+import androidx.compose.ui.platform.LocalDensity
 import androidx.compose.ui.platform.testTag
 import androidx.compose.ui.res.painterResource
 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.SpanStyle
+import androidx.compose.ui.text.TextStyle
+import androidx.compose.ui.text.style.BaselineShift
 import androidx.compose.ui.text.style.TextOverflow
+import androidx.compose.ui.unit.Density
+import androidx.compose.ui.unit.isUnspecified
 import androidx.compose.ui.unit.IntSize
 import androidx.lifecycle.compose.collectAsStateWithLifecycle
 import com.example.research.R
@@ -48,6 +55,72 @@ fun ArticleRoute(
     )
 }
 
+private const val MERGED_BLOCK_MAX_CHARS = 1500
+private const val MERGED_BLOCK_MAX_ITEMS = 12
+
+private fun mergeDslBlocks(
+    blocks: List<DslBlock>,
+    density: Density,
+    baseStyle: TextStyle,
+): List<DslBlock> {
+    if (blocks.size <= MERGED_BLOCK_MAX_ITEMS) return blocks
+    val gapShift = with(density) {
+        val fontPx =
+            if (baseStyle.fontSize.isUnspecified) 0f else baseStyle.fontSize.toPx()
+        if (fontPx > 0f) 8.dp.toPx() / (0.534f * fontPx) else 0.94f
+    }
+    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 && !blocks[j + 1].continuesPrevious) {
+                val shiftPos = builder.length
+                builder.append('​')
+                builder.addStyle(
+                    SpanStyle(baselineShift = BaselineShift(-gapShift)),
+                    shiftPos,
+                    shiftPos + 1
+                )
+            }
+            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?,
@@ -123,6 +196,14 @@ fun ArticleContent(
             val isHeavyArticle =
                 articleState.dslContent.size > HEAVY_ARTICLE_BLOCK_COUNT
 
+            val density = LocalDensity.current
+            val displayBlocks = remember(
+                articleState.dslContent,
+                density,
+                baseTextStyle
+            ) {
+                mergeDslBlocks(articleState.dslContent, density, baseTextStyle)
+            }
             val articleList: @Composable (Modifier) -> Unit = { listModifier ->
                 LazyColumn(
                     state = listState,
@@ -134,15 +215,14 @@ 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
+                            displayBlocks.getOrNull(index + 1)?.continuesPrevious == true
                         val textContent = @Composable {
                             Box(
                                 modifier = Modifier