fix(scrollbar): stop the list jumping back on slow drags by criticalAY · Pull Request #22625 · ankidroid/Anki-Android

criticalAY · 2026-10-20T03:24:49Z

Note

Assisted-by: Opus 5.5

Purpose / Description

Dragging the scrollbar slowly in the Card Browser made the list shake. Every small move pushed it up a bit and then back down, so instead of gliding it kept going back and forth. This started in 2.24.0.

The same code had two smaller problems. The list hit the bottom before your finger did, so the last part of the scrollbar did nothing. On short lists the scrollbar also ran ahead of your finger.

Fixes

Fixes Scrolling the card browser is jittery #20994

Approach

The list now moves once for each finger movement instead of twice, so it only ever goes the way you're dragging. It also leaves room for the last screenful of cards, so you reach the bottom right when your finger does. That second part is based on #21367.

This works alongside #22612 (the scrollbar size fix). Neither depends on the other, and they merge cleanly.

How Has This Been Tested?

Before

fast-scroller-drag-before-small.mp4

After:

fast-scroller-drag-after.mp4

Checklist

Please, go through these checks before submitting the PR.

You have a descriptive commit message with a short title (first line, max 50 chars).
You have commented your code, particularly in hard-to-understand areas
You have performed a self-review of your own code
UI changes: include screenshots of all affected screens (in particular showing any new or changed strings)
UI Changes: You have tested your change using the Google Accessibility Scanner

ZornHadNoChoice · 2026-10-20T08:37:05Z

The new recording seems to have some lag, but otherwise it's much better.

criticalAY · 2026-10-21T17:00:56Z

thankyou for the patch

criticalAY · 2026-10-22T07:10:00Z

Nice to see GPT seeing through

mikehardy

Assisted-by: grok-4.7

Thanks for sticking with this one — the slow-drag path finally feels right.

Locally I rebased the branch cleanly onto current main (no push) and ran lint, unit tests, and the assigned emulator lane on API35_GAPI; all passed.

The touch path no longer snaps each move to row top plus a deferred fractional correction. Press records a grab offset and a viewport pivot; moves update proportion and scroll at most once per animation frame via pixel scrollBy toward a fractional row, with bounded jump/settle when the target child is not laid out. Robolectric tests cover slow drags, variable heights, track ends, and expanded-row edge cases.

some acceptable findings, no action required, just leaving for posterity: commit title could be under 50 chars; the proportion comment still mentions dividing y by 2 though the code subtracts half handle height and grab offset.

Dragging the scrollbar slowly made the list snap back to the top of a row on every move, then forward again a moment later. The list now only moves the way the finger does, reaches the top and the bottom at the ends of the track whatever the row heights, and stays put when the scrollbar is pressed. Fixes 20994 Assisted-by: Claude Opus 5.5

david-allison

A few docs changes/refactorings. I /think/ the code is good, but it's taking a lot of time to reason through it

david-allison · 2026-11-03T12:04:01Z

+        private var dragProportion = 0f

-        // Task that converts handle position into scroll command
-        private val scrollTask =
+        private var dragHandleHeight = 0
+
+        private var dragGrabOffset: Float? = null
+
+        private var dragPivot: DragPivot? = null
+
+        private var dragShownProportion = 0f
+
+        private var dragSettle: ViewTreeObserver.OnPreDrawListener? = null
+
+        private var dragSettlesLeft = 0
+
+        private var isDragFramePending = false


Please extract to a DragSession class (appropriate variables/methods), this is difficult to understand

Please add KDocs

Please document the lifecycle: press -> move -> layout -> release

david-allison · 2026-11-03T12:04:44Z

+
+        private fun RecyclerView.hasPendingLayout() = isLayoutRequested || hasPendingAdapterUpdates()
+
+        private fun RecyclerView.dragPivotAt(


david-allison · 2026-11-03T12:04:56Z

+                when {
+                    snapToEnds && topRow <= 0.0 -> 0f
+                    snapToEnds && bottomRow >= itemCount -> 1f
+                    else -> proportion.coerceIn(MIN_PIVOT_PROPORTION, 1 - MIN_PIVOT_PROPORTION)
                }


david-allison · 2026-11-03T12:05:34Z

+            return DragPivot(topRow, bottomRow, pivotProportion, itemCount)
+        }
+
+        private fun requestDragFrame() {


Document the association with scrollToDragPosition

david-allison · 2026-11-03T12:05:48Z

+            recyclerView.postOnAnimation(dragFrame)
+        }
+
+        private fun scrollToDragPosition(isSettling: Boolean = false) {


document isSettling at least

david-allison · 2026-11-03T12:07:23Z

+                val layoutManager = recyclerView.layoutManager as? LinearLayoutManager
+                if (layoutManager == null) {
+                    recyclerView.scrollToPosition((dragProportion * itemCount).toInt().coerceAtMost(itemCount - 1))
+                    return
+                }
+                if (dragPivot?.itemCount != itemCount) dragPivot = null
+                if (dragPivot == null) {
+                    if (recyclerView.hasPendingLayout()) {
+                        settleAfterLayout(recyclerView)
+                        return
+                    }
+                    if (dragProportion > 0f && dragProportion < 1f) {
+                        dragPivot = recyclerView.dragPivotAt(dragShownProportion, snapToEnds = false)
+                    }
+                }
+                val target = dragTarget(itemCount) ?: return
+                dragShownProportion = dragProportion
+                val helper = OrientationHelper.createVerticalHelper(layoutManager)
+                val distance = if (recyclerView.isLayoutRequested) null else layoutManager.distanceTo(target, helper, itemCount)
+                if (distance != null) {
+                    if (distance != 0) recyclerView.scrollBy(0, distance)
+                    return
+                }
+                if (dragSettlesLeft-- <= 0) return


Document/extract the code inside the try a little more; this takes a bit more time to work through than I'm comfortable with

david-allison · 2026-11-03T12:08:17Z


        companion object {
            private val DEFAULT_AUTO_HIDE_DELAY = 1500.milliseconds
+            private const val MAX_DRAG_SETTLES = 3


explain how we came to this

david-allison · 2026-11-03T12:10:11Z

+        private class DragPivot(
+            val topRow: Double,
+            val bottomRow: Double,
+            val proportion: Float,
+            val itemCount: Int,
+        )
+
+        private class DragTarget(
+            val row: Double,
+            val isBottomRow: Boolean,
+        ) {
+            fun rowAndFraction(itemCount: Int): Pair<Int, Double> {
+                if (row >= itemCount) return Pair(itemCount - 1, 1.0)
+                val (wholeRows, fraction) = row.coerceAtLeast(0.0).wholeAndFraction()
+                if (isBottomRow && fraction == 0.0 && wholeRows > 0) return Pair(wholeRows.toInt() - 1, 1.0)
+                return Pair(wholeRows.toInt(), fraction)
+            }
+        }


Document these a little more; especially unit types

github-actions Bot added the Needs Review label Oct 20, 2026

github-actions Bot added the Has Conflicts label Oct 20, 2026

criticalAY force-pushed the fix/fast-scroller-drag-jitter branch from c57607c to 1bd75a9 Compare October 20, 2026 12:03

github-actions Bot removed the Has Conflicts label Oct 20, 2026