diff --git a/app/androidApp/src/main/kotlin/com/example/aiapp/TranscriptScroll.kt b/app/androidApp/src/main/kotlin/com/example/aiapp/TranscriptScroll.kt
index 5db4920..bb81e4a 100644
--- a/app/androidApp/src/main/kotlin/com/example/aiapp/TranscriptScroll.kt
+++ b/app/androidApp/src/main/kotlin/com/example/aiapp/TranscriptScroll.kt
@@ -98,9 +98,35 @@ class TranscriptScroll(internal val scroll: ScrollState) {
     }
 
     internal fun height(seq: Long, height: Int) {
-        if (heights.put(seq, height) != height) topsStale = true
+        val had = heights.put(seq, height)
+        if (had == height) return
+        measuredTotal += height - (had ?: 0)
+        if (had == null) measuredCount++
+        topsStale = true
     }
 
+    private var measuredTotal = 0L
+    private var measuredCount = 0
+
+    /**
+     * How tall to assume a row is before anything has measured it.
+     *
+     * A row that has just been paged in has no height, and something has to stand in for one or it
+     * cannot be placed at all. This used to be answered by keeping every unmeasured row built --
+     * which meant a page of history standing up seventeen screens of markdown inside a single
+     * frame, measured on a Pixel 9 Pro XL as a hundred milliseconds in one go, with the frame after
+     * it unable to start. An estimate lets a paged-in row be a spacer like any other distant row,
+     * and be built when the reader actually comes near it.
+     *
+     * The average of what has been measured, because the average row in a conversation is a good
+     * guess at the next one and a constant is not: these run from a one-line note to a screenful.
+     * Being wrong is cheap and self-correcting -- the estimate is only used above the viewport,
+     * where the content hangs from its far end, so a correction there moves nothing on screen.
+     */
+    private fun assumed(seq: Long): Int =
+        heights[seq]
+            ?: if (measuredCount > 0) (measuredTotal / measuredCount).toInt() else ROW_GUESS
+
     /**
      * The running total of every row's top edge, recomputed at most once per frame and only after
      * something has actually changed height.
@@ -115,7 +141,7 @@ class TranscriptScroll(internal val scroll: ScrollState) {
         var y = padTop
         order.forEachIndexed { index, seq ->
             out[index] = y
-            y += (heights[seq] ?: 0) + spacing
+            y += assumed(seq) + spacing
         }
         tops = out
         topsStale = false
@@ -181,7 +207,7 @@ class TranscriptScroll(internal val scroll: ScrollState) {
         var y = padTop
         for (s in order) {
             if (s == seq) return y
-            y += (heights[s] ?: return null) + spacing
+            y += assumed(s) + spacing
         }
         return null
     }
@@ -225,12 +251,14 @@ class TranscriptScroll(internal val scroll: ScrollState) {
      * of the height it was last measured at, so the transcript's total height does not change and
      * nothing under the reader moves.
      *
-     * A row that has never been measured has no height to stand in for it and is kept whatever its
-     * distance -- that is a row that has just been paged in, and it is about to be looked at.
+     * A row that has never been measured stands in at the average of those that have, so it is
+     * placed and judged like any other; see [assumed]. Keeping every unmeasured row instead is what
+     * made a page of history land as one hundred-millisecond frame -- seventeen screens of markdown
+     * shaped at once, because "not measured yet" described the whole page.
      */
     fun retains(seq: Long): Boolean {
         val index = rowIndex[seq] ?: return true
-        val height = heights[seq] ?: return true
+        val height = assumed(seq)
         refreshTops()
         val top = tops.getOrNull(index) ?: return true
         val viewportTop = scroll.maxValue - scroll.value
@@ -240,7 +268,7 @@ class TranscriptScroll(internal val scroll: ScrollState) {
     }
 
     /** The height a row was last measured at, for the spacer that stands in for it. */
-    fun heightOf(seq: Long): Int = heights[seq] ?: 0
+    fun heightOf(seq: Long): Int = assumed(seq)
 
     /** The height of the visible area, 0 until the first measurement. */
     val viewport: Int
@@ -259,7 +287,7 @@ class TranscriptScroll(internal val scroll: ScrollState) {
         for (s in order) {
             if (y > top) break
             found = s to y
-            y += (heights[s] ?: return null) + spacing
+            y += assumed(s) + spacing
         }
         return found?.let { (seq, rowTop) -> ScrollAnchor(seq, top - rowTop) }
     }
@@ -429,3 +457,6 @@ private fun RetainedRow(
 
 /** How far either side of the screen a row stays built; see [TranscriptScroll.retains]. */
 private const val RETAIN_SCREENS = 8
+
+/** What a row is assumed to be worth before any of them have been measured. */
+private const val ROW_GUESS = 800