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

Removed calculation scroll thomb position and size based on RecyclerView.computeVerticalScrollRange() / computeVerticalScrollOffset(), because it depended on scroll position and recalculated while scrolling and made glitch.
This PR:

caches the thumb height and scroll range so the thumb keeps a stable size and scale while scrolling
tracks the thumb position using accumulated real scroll deltas (dy) instead of recalculating it from RecyclerView's estimated scroll offset on every layout
snaps the accumulated offset to the exact start/end when RecyclerView reports that the list is at an edge
resets cached scroll metrics when adapter data, list width, or viewport height changes
applies the same custom fast scroller to the tags dialog, replacing the native RecyclerView scrollbar there

I also add the changed scroller to fix tags glitch.

How Has This Been Tested?

Manual UI test.

Before:

telegram-cloud-document-2-5373303824473696425.mp4

telegram-cloud-document-2-5377809133498049085.mp4

After

telegram-cloud-document-2-5379845175629623475.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

…humb size change on tap to open menu)

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.

amoniaka-knabino added 6 commits July 12, 2026 22:17

Stabilize browser scrollbar thumb size

addf2a9

Try stabilize scrollbar thumb position

0bd67a1

refactor

a93b8ed

add comment

981be0e

add comment to getCachedVisibleItemCount and getCachedHandleHeight funcs

f508abc

add comment to getScrollProportion

99e2579

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

Switch tags to RecyclerFastScroller

82eba9e

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

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

Merge branch 'main' into fix-scroll-thomb

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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

criticalAY added the Needs Author Reply Waiting for a reply from the original author label Jul 13, 2026

amoniaka-knabino added 2 commits July 13, 2026 18:07

apply criticalay patch + remove resolve confict artifacts

a54f144

null safe to attachFastScroller

4a38ab9

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amoniaka-knabino added 2 commits July 14, 2026 10:30

apply criticalAY patch 2

680e705

cache scroll offset and range

bbef5ae

add more comments

070a523

amoniaka-knabino added 2 commits July 15, 2026 09:21

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

16c681c

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

a34c549

…humb size change on tap to open menu)

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Purpose / Description

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