From e64d20792b589cf9aa5e3b5d9b66c5679649c4de Mon Sep 17 00:00:00 2001
From: Petyo Ivanov <underlog@gmail.com>
Date: Fri, 6 Aug 2021 09:27:47 +0300
Subject: [PATCH] fix(): handle very tall last item in reverse mode

---
 src/scrollToIndexSystem.ts | 21 ++++++++++++++++++++-
 test/listSystem.test.ts    | 13 ++++++++-----
 2 files changed, 28 insertions(+), 6 deletions(-)

diff --git a/src/scrollToIndexSystem.ts b/src/scrollToIndexSystem.ts
index bb59b7d71..77e7e7aa7 100644
--- a/src/scrollToIndexSystem.ts
+++ b/src/scrollToIndexSystem.ts
@@ -88,6 +88,7 @@ export const scrollToIndexSystem = u.system(
             cleanup()
             if (listChanged) {
               u.publish(scrollToIndex, location)
+            } else {
             }
           }
 
@@ -103,7 +104,7 @@ export const scrollToIndexSystem = u.system(
               retry(listChanged)
             })
           } else {
-            unsubscribeNextListRefresh = u.handleNext(listRefresh, retry)
+            unsubscribeNextListRefresh = u.handleNext(u.pipe(listRefresh, accumulateChange(0)), retry)
           }
 
           // if the scroll jump is too small, the list won't get rerendered.
@@ -127,3 +128,21 @@ export const scrollToIndexSystem = u.system(
   u.tup(sizeSystem, domIOSystem),
   { singleton: true }
 )
+
+function accumulateChange<T>(interval: number): u.Operator<T> {
+  let currentValue: T | undefined
+  let timeout: any
+
+  return (done) => (value) => {
+    currentValue = currentValue || value
+
+    if (timeout) {
+      return
+    }
+
+    timeout = setTimeout(() => {
+      timeout = undefined
+      done(currentValue!)
+    }, interval)
+  }
+}
diff --git a/test/listSystem.test.ts b/test/listSystem.test.ts
index 1854f9f72..ec641e8a3 100644
--- a/test/listSystem.test.ts
+++ b/test/listSystem.test.ts
@@ -174,7 +174,7 @@ describe('list engine', () => {
       })
     })
 
-    it('readjusts once when new sizes are reported', () => {
+    it('readjusts once when new sizes are reported', (done) => {
       const DEVIATION = 20
       publish(sti, { index: INDEX, align: 'end' })
 
@@ -185,10 +185,13 @@ describe('list engine', () => {
 
       publish(sr, [{ startIndex: INDEX - 1, endIndex: INDEX - 1, size: SIZE + DEVIATION }])
 
-      expect(sub).toHaveBeenCalledWith({
-        top: INDEX * SIZE - VIEWPORT + SIZE + DEVIATION,
-        behavior: 'auto',
-      })
+      setTimeout(() => {
+        expect(sub).toHaveBeenCalledWith({
+          top: INDEX * SIZE - VIEWPORT + SIZE + DEVIATION,
+          behavior: 'auto',
+        })
+        done()
+      }, 20)
     })
   })