Fix scroll thomb length changing on cards /tags lists of different size rows on bug by amoniaka-knabino · Pull Request #21367 · ankidroid/Anki-Android

amoniaka-knabino · 2026-07-12T17:24:09Z

Purpose / Description

Fix scroll UI bug: before length changed while scrolling list of rows of different length and looked glitchy, I want to make it smooth for better view.

Fixes

Part of Scrollbar thumb changes length when scolling #19480

Approach

RecyclerView.computeVerticalScrollRange() and computeVerticalScrollOffset() are estimates based on currently visible rows. With variable-height rows, these estimates can change while scrolling, which causes the scrollbar thumb to resize or jump.

This PR:

caches the initial scroll range and thumb height so their scale remains stable while scrolling
initializes the scroll offset from RecyclerView once, then tracks it using the actual pixel deltas (dy)
invalidates cached metrics when the adapter structure, list width, or viewport height changes, while ignoring content-only row updates that do not change the list structure
calibrates the cached range after observing a complete normal scroll from the real top to the real bottom
approaches the end of an uncalibrated range smoothly and animates the final correction when normal scrolling reaches the real bottom
handles thumb dragging separately from normal scrolling, keeping the thumb under the user's finger and reserving space for the final visible rows
avoids an unnecessary full row refresh when opening the note editor from single-select mode, preventing cached scroll metrics from being reset
applies the same custom fast scroller to the tags dialog instead of its native RecyclerView scrollbar
makes attachFastScroller() a safe no-op when the requested scroller is not present in the layout

How Has This Been Tested?

Manual UI test.

Before:

Current main performance.

telegram-cloud-document-2-5397669861138671532.mp4

After

Current branch performance.

FILE.2026-07-19.13.30.59-compressed.mp4

Learning (optional, can help others)

Describe the research stage

Links to blog posts, patterns, libraries or addons used to solve this problem

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

sanjaysargam

Thanks @amoniaka-knabino
Consider squashing the documentation and refactor commits for cleaner history

sanjaysargam · 2026-07-13T07:46:03Z

+        private var cachedHandleHeightItemCount = RecyclerView.NO_POSITION
+        private var cachedHandleHeightBarHeight = 0
+
+        private fun resetCachedHandleHeight() {


document it

sanjaysargam · 2026-07-13T07:46:12Z

            handle.layout(handle.left, y.toInt(), handle.right, y.toInt() + calculatedHandleHeight)
        }

+        private fun getCachedHandleHeight(


document it

sanjaysargam · 2026-07-13T07:59:28Z

+    <RelativeLayout
        android:layout_width="match_parent"
-        android:layout_height="wrap_content"
+        android:layout_height="match_parent"


could you explain why ?

This wrapper needs to fill the available list area because it now contains both the RecyclerView and the overlay fast scroller. Keeping wrap_content would size the wrapper from its children instead of the available viewport, so the scroller track could end up with the wrong height.

sanjaysargam · 2026-07-13T08:01:03Z


            tagsArrayAdapter = TagsArrayAdapter(tags) { binding.root.showMaxTagSelectedNotice(tags) }
            binding.tagsList.adapter = tagsArrayAdapter
+            binding.tagsList.attachFastScroller(R.id.tags_scroller)


What is scroller ID is not found ? It may crash
worth adding a null safe

Good point, thank you. I made attachFastScroller null safe

amoniaka-knabino · 2026-07-13T13:28:54Z

The item-index approach is the right call here, and it now lines up with the drag handler too. I put a small follow-up patch(please it test) on top of yours: it collapses the caching into one keyed value, brings back the hide-when-the-list-fits behaviour, and fixes two edge cases (Card Browser attaches the scroller before its adapter, so the data observer never registers and the thumb size goes stale after a Truncate toggle, and an empty search leaves a full-height thumb behind). I also pulled the math into two small pure functions with unit tests. Feel free to take whatever is useful.

@criticalAY Thank you for the patch!

I tested it, because of smaller scroll thomb size, position looks more glitchy after the patch (please look at videos below, especially the cards video). How do you think, it's OK or we need to change it somehow?

telegram-cloud-document-2-5380060933311732952.mp4

telegram-cloud-document-2-5380060933311732950.mp4

amoniaka-knabino · 2026-07-14T06:14:01Z

@criticalAY I tried your second patch as it is, but glitch still was there. After I added accumulatedScrollOffset and it gets better. Position is smooth everywhere except start: scroll thomb "jumps" to start position when you scroll from end to start, but it's not really irritating, I think.

after patch 2:

telegram-cloud-document-2-5379845175629623469.mp4

after accumulatedScrollOffset: (scroll jump at 0:22)

telegram-cloud-document-2-5379845175629623475.mp4

ZornHadNoChoice · 2026-07-14T07:28:33Z

The scrolling in this last recording looks really nice, but:

The snapping at the ends isn't great.
The thumb height changes at around 0:07 when opening the note editor.

If it's not too much to ask, consider also testing the effects of this code on #20994. You can use the same deck I shared in #19480 (comment) to test both issues (make sure to import with scheduling information to get the flags; they help see the other issue).

criticalAY · 2026-07-14T17:34:03Z

@amoniaka-knabino Can this be rebased? Or if you are not sure how then I can help here, just let me know

amoniaka-knabino · 2026-07-15T03:11:02Z

If it's not too much to ask, consider also testing the effects of this code on #20994. You can use the same deck I shared in #19480 (comment) to test both issues (make sure to import with scheduling information to get the flags; they help see the other issue).

Thank you for noticing! I will try to fix it.

As I see, current version works OK on this collections, except scrolling in the end. At least, I can't see glitch and size changing while opening the editor on such small scroll thumb size.

telegram-cloud-document-2-5382096975443304703-compressed.mp4

telegram-cloud-document-2-5382096975443304705-compressed.mp4

telegram-cloud-document-2-5382096975443304708.mp4

telegram-cloud-document-2-5382096975443304707.mp4

ZornHadNoChoice · 2026-07-15T09:12:44Z

The problem at the ends is worse with this bigger deck. Also, for the other issue, you need to grab the scrollbar thumb and slowly drag it.

amoniaka-knabino · 2026-07-15T11:19:59Z

@ZornHadNoChoice I added scroll calibration and it become smooth.
Also I removed requestLayout() in onItemRangeChanged(), so opening card menu don't trigger thumb size change.

2026-07-15.16.11.14-compressed-under-10mb.mp4

UPD: I also tested grab&drag, it still have jump in the end. I'll see what can I do with it.

telegram-cloud-document-2-5384348775256988729.1.mp4

ZornHadNoChoice · 2026-07-16T05:46:11Z

It's definitely much smoother, but the thumb moves slower at the lower end.

amoniaka-knabino · 2026-07-16T09:07:19Z

It's definitely much smoother, but the thumb moves slower at the lower end.

Thumb moves slower at the lower end because of calibration. It's hard to be fixed, because in the beginning of the scroll we don't know exactly height of all rows, so our scroll size and speed is not really precise. When we are close to end but still not at the last point, we recalculate scroll speed to let scroll thumb move until we really scroll to the last element.

…humb size change on tap to open menu)

amoniaka-knabino · 2026-07-19T09:05:36Z

Hi @ZornHadNoChoice @criticalAY @sanjaysargam !

I fixed things that you wrote to me in the comments and now I am satisfied with current scroll thumb performance. I hope this version of fix is final and can be merged.

You can see current version performance in the description of PR.

amoniaka-knabino marked this pull request as ready for review July 12, 2026 17:45

amoniaka-knabino changed the title ~~Fix scroll thomb~~ Fix scroll thomb length changing on lists of different size rows bug Jul 12, 2026

amoniaka-knabino changed the title ~~Fix scroll thomb length changing on lists of different size rows bug~~ Fix scroll thomb length changing on lists of different size rows on cards bug Jul 12, 2026

amoniaka-knabino changed the title ~~Fix scroll thomb length changing on lists of different size rows on cards bug~~ Fix scroll thomb length changing on cards /tags lists of different size rows on bug Jul 12, 2026

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sanjaysargam requested changes Jul 13, 2026

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github-actions Bot removed the Has Conflicts label Jul 13, 2026

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amoniaka-knabino added 11 commits July 19, 2026 13:18

Stabilize browser scrollbar thumb size

b894c80

Try stabilize scrollbar thumb position

f07f297

refactor

1366800

add comment

c3a0afd

add comment to getCachedVisibleItemCount and getCachedHandleHeight funcs

16fc3ae

add comment to getScrollProportion

ffc4693

Switch tags to RecyclerFastScroller

83ae672

apply criticalay patch + remove resolve confict artifacts

53bd12e

null safe to attachFastScroller

69a3cff

apply criticalAY patch 2

f2d0c71

cache scroll offset and range

e414bf6

amoniaka-knabino added 6 commits July 19, 2026 13:19

add more comments

b9fe9bd

add scroll calibration to make scroll in the end correct and smooth

9e1511c

update only on content change, ignore itemrangechange (fixes scroll t…

ffc4ff3

…humb size change on tap to open menu)

fix dgrab&drag scroll jump

fea2103

smaller border for end calibration

55463a3

don't reset cache on card menu open

8606f14

amoniaka-knabino force-pushed the fix-scroll-thomb branch from a1619ba to 8606f14 Compare July 19, 2026 08:34

add comments

178fd46

amoniaka-knabino requested review from criticalAY and sanjaysargam July 19, 2026 12:59

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