Animate reaction counts (#904)

desktop/src/features/messages/ui/MessageReactions.tsx

@@ -6,6 +6,7 @@ import type { TimelineReaction } from "@/features/messages/types";
 import { cn } from "@/shared/lib/cn";
 import { emojiDisplayName } from "@/shared/lib/emojiName";
 import { rewriteRelayUrl } from "@/shared/lib/mediaUrl";
+import { AnimatedCount } from "@/shared/ui/AnimatedCount";
 import {
   isPositiveEmojiParticle,
   useEmojiBurst,
@@ -433,7 +434,10 @@ function ReactionPill({
         type="button"
       >
         <EmojiGlyph reaction={reaction} className={REACTION_GLYPH_CLASSES} />
-        <span className="text-muted-foreground">{reaction.count}</span>
+        <AnimatedCount
+          className="text-muted-foreground"
+          value={reaction.count}
+        />
       </button>
     );
   }
@@ -463,7 +467,10 @@ function ReactionPill({
               reaction={reaction}
               className={REACTION_GLYPH_CLASSES}
             />
-            <span className="text-muted-foreground">{reaction.count}</span>
+            <AnimatedCount
+              className="text-muted-foreground"
+              value={reaction.count}
+            />
           </button>
         </span>
       </PopoverTrigger>

desktop/src/features/pulse/ui/NoteCard.tsx

@@ -14,6 +14,7 @@ import { useNoteByIdQuery } from "@/features/pulse/hooks";
 import { getReplyParent, noteSnippet } from "@/features/pulse/lib/replies";
 import type { UserNote } from "@/shared/api/socialTypes";
 import type { ChannelMember, UserProfileSummary } from "@/shared/api/types";
+import { AnimatedCount } from "@/shared/ui/AnimatedCount";
 import { Markdown } from "@/shared/ui/markdown";
 import { Tooltip, TooltipContent, TooltipTrigger } from "@/shared/ui/tooltip";
 import { UserAvatar } from "@/shared/ui/UserAvatar";
@@ -149,9 +150,7 @@ export function NoteCard({
   const activeActionClass = "text-primary";
   const countPlaceholder = <span aria-hidden className="w-2.5" />;
   const reactionCountLabel =
-    reactionCount > 0 ? (
-      <span className="tabular-nums">{reactionCount}</span>
-    ) : null;
+    reactionCount > 0 ? <AnimatedCount value={reactionCount} /> : null;
   const currentUserAvatarUrl = currentUserProfile?.avatarUrl ?? null;
   const replyParentId = getReplyParent(note);
 
@@ -224,7 +223,7 @@ export function NoteCard({
                 <button
                   aria-label={isUpvoted ? "Unlike" : "Like"}
                   aria-pressed={isUpvoted}
-                  className={`${actionButtonClass} ${isUpvoted ? activeActionClass : ""} disabled:cursor-not-allowed disabled:opacity-45`}
+                  className={`${actionButtonClass} ${isUpvoted ? activeActionClass : ""} disabled:opacity-45`}
                   disabled={isUpvotePending}
                   onClick={() => {
                     if (!isUpvotePending) {

desktop/src/shared/styles/globals.css

@@ -97,6 +97,129 @@
   -webkit-user-select: none;
 }
 
+.sprout-animated-count {
+  display: inline-block;
+  font-kerning: none;
+  font-variant-numeric: tabular-nums;
+  line-height: 1;
+  white-space: nowrap;
+}
+
+.sprout-animated-count__motion {
+  display: inline-flex;
+  line-height: 1;
+}
+
+.sprout-animated-count__static {
+  display: none;
+}
+
+.sprout-animated-count__slot {
+  display: inline-block;
+  height: 1em;
+  line-height: 1;
+}
+
+.sprout-animated-count__slot--digit {
+  overflow: hidden;
+  text-align: center;
+  width: 1ch;
+}
+
+.sprout-animated-count__reel {
+  display: flex;
+  flex-direction: column;
+  height: 2em;
+  line-height: 1;
+  will-change: transform;
+}
+
+.sprout-animated-count__reel > span {
+  display: block;
+  height: 1em;
+  line-height: 1;
+}
+
+.sprout-animated-count__slot[data-direction="up"] .sprout-animated-count__reel {
+  animation: sprout-animated-count-roll-up 260ms cubic-bezier(0.2, 0.8, 0.2, 1)
+    both;
+}
+
+.sprout-animated-count__slot[data-direction="down"]
+  .sprout-animated-count__reel {
+  animation: sprout-animated-count-roll-down 260ms
+    cubic-bezier(0.2, 0.8, 0.2, 1) both;
+}
+
+.sprout-animated-count__symbol {
+  display: inline-block;
+  line-height: 1;
+}
+
+.sprout-animated-count__symbol[data-direction="up"] {
+  animation: sprout-animated-count-symbol-up 220ms
+    cubic-bezier(0.2, 0.8, 0.2, 1) both;
+}
+
+.sprout-animated-count__symbol[data-direction="down"] {
+  animation: sprout-animated-count-symbol-down 220ms
+    cubic-bezier(0.2, 0.8, 0.2, 1) both;
+}
+
+@keyframes sprout-animated-count-roll-up {
+  from {
+    transform: translateY(0);
+  }
+
+  to {
+    transform: translateY(-1em);
+  }
+}
+
+@keyframes sprout-animated-count-roll-down {
+  from {
+    transform: translateY(-1em);
+  }
+
+  to {
+    transform: translateY(0);
+  }
+}
+
+@keyframes sprout-animated-count-symbol-up {
+  from {
+    opacity: 0;
+    transform: translateY(0.25em);
+  }
+
+  to {
+    opacity: 1;
+    transform: translateY(0);
+  }
+}
+
+@keyframes sprout-animated-count-symbol-down {
+  from {
+    opacity: 0;
+    transform: translateY(-0.25em);
+  }
+
+  to {
+    opacity: 1;
+    transform: translateY(0);
+  }
+}
+
+@media (prefers-reduced-motion: reduce) {
+  .sprout-animated-count__motion {
+    display: none;
+  }
+
+  .sprout-animated-count__static {
+    display: inline;
+  }
+}
+
 /* ── Tiptap rich-text composer ──────────────────────────────────────── */
 .rich-text-composer .tiptap {
   outline: none;

desktop/src/shared/ui/AnimatedCount.tsx

@@ -0,0 +1,183 @@
+import * as React from "react";
+
+import { cn } from "@/shared/lib/cn";
+import {
+  formatAnimatedCount,
+  getAnimatedCountSlots,
+  normalizeAnimatedCount,
+} from "@/shared/ui/animatedCountParts";
+
+type CountDirection = -1 | 0 | 1;
+
+type AnimatedCountTransition = {
+  current: string;
+  direction: CountDirection;
+  previous: string;
+  version: number;
+};
+
+type AnimatedCountProps = {
+  className?: string;
+  value: number;
+};
+
+const ANIMATED_COUNT_SETTLE_DELAY_MS = 260;
+
+function directionFor(previous: number, current: number): CountDirection {
+  if (current > previous) return 1;
+  if (current < previous) return -1;
+  return 0;
+}
+
+function renderSlotText(value: string) {
+  return value || "\u00A0";
+}
+
+function AnimatedCountSlot({
+  current,
+  direction,
+  isDigit,
+  previous,
+  version,
+}: {
+  current: string;
+  direction: CountDirection;
+  isDigit: boolean;
+  previous: string;
+  version: number;
+}) {
+  if (direction === 0 || previous === current) {
+    return (
+      <span
+        className={cn(
+          "sprout-animated-count__slot",
+          isDigit && "sprout-animated-count__slot--digit",
+        )}
+      >
+        {renderSlotText(current)}
+      </span>
+    );
+  }
+
+  if (!isDigit) {
+    return (
+      <span
+        className="sprout-animated-count__symbol"
+        data-direction={direction > 0 ? "up" : "down"}
+        key={`${version}-${previous}-${current}`}
+      >
+        {renderSlotText(current)}
+      </span>
+    );
+  }
+
+  return (
+    <span
+      className="sprout-animated-count__slot sprout-animated-count__slot--digit"
+      data-direction={direction > 0 ? "up" : "down"}
+    >
+      <span
+        className="sprout-animated-count__reel"
+        key={`${version}-${previous}-${current}`}
+      >
+        {direction > 0 ? (
+          <>
+            <span>{renderSlotText(previous)}</span>
+            <span>{renderSlotText(current)}</span>
+          </>
+        ) : (
+          <>
+            <span>{renderSlotText(current)}</span>
+            <span>{renderSlotText(previous)}</span>
+          </>
+        )}
+      </span>
+    </span>
+  );
+}
+
+export function AnimatedCount({ className, value }: AnimatedCountProps) {
+  const normalizedValue = normalizeAnimatedCount(value);
+  const formattedValue = formatAnimatedCount(normalizedValue);
+  const previousValueRef = React.useRef(normalizedValue);
+  const [transition, setTransition] = React.useState<AnimatedCountTransition>(
+    () => ({
+      current: formattedValue,
+      direction: 0,
+      previous: formattedValue,
+      version: 0,
+    }),
+  );
+
+  React.useLayoutEffect(() => {
+    setTransition((currentTransition) => {
+      if (currentTransition.current === formattedValue) {
+        previousValueRef.current = normalizedValue;
+        return currentTransition;
+      }
+
+      const direction = directionFor(previousValueRef.current, normalizedValue);
+      previousValueRef.current = normalizedValue;
+
+      return {
+        current: formattedValue,
+        direction,
+        previous: currentTransition.current,
+        version: currentTransition.version + 1,
+      };
+    });
+  }, [formattedValue, normalizedValue]);
+
+  React.useEffect(() => {
+    if (
+      transition.direction === 0 ||
+      transition.previous === transition.current
+    ) {
+      return;
+    }
+
+    const timeoutId = window.setTimeout(() => {
+      setTransition((currentTransition) => {
+        if (currentTransition.version !== transition.version) {
+          return currentTransition;
+        }
+
+        return {
+          ...currentTransition,
+          direction: 0,
+          previous: currentTransition.current,
+        };
+      });
+    }, ANIMATED_COUNT_SETTLE_DELAY_MS);
+
+    return () => window.clearTimeout(timeoutId);
+  }, [
+    transition.current,
+    transition.direction,
+    transition.previous,
+    transition.version,
+  ]);
+
+  const slots = getAnimatedCountSlots(transition.previous, transition.current);
+
+  return (
+    <span className={cn("sprout-animated-count", className)}>
+      <span className="sr-only">{transition.current}</span>
+      <span aria-hidden className="sprout-animated-count__motion">
+        {slots.map((slot) => (
+          <AnimatedCountSlot
+            current={slot.current}
+            direction={transition.direction}
+            isDigit={slot.isDigit}
+            key={`${transition.version}-${slot.place}`}
+            previous={slot.previous}
+            version={transition.version}
+          />
+        ))}
+      </span>
+      <span aria-hidden className="sprout-animated-count__static">
+        {transition.current}
+      </span>
+    </span>
+  );
+}

desktop/src/shared/ui/animatedCountParts.test.mjs

@@ -0,0 +1,34 @@
+import assert from "node:assert/strict";
+import test from "node:test";
+
+import {
+  formatAnimatedCount,
+  getAnimatedCountSlots,
+  normalizeAnimatedCount,
+} from "./animatedCountParts.ts";
+
+test("normalizeAnimatedCount clamps to displayable whole counts", () => {
+  assert.equal(normalizeAnimatedCount(4.9), 4);
+  assert.equal(normalizeAnimatedCount(-1), 0);
+  assert.equal(normalizeAnimatedCount(Number.NaN), 0);
+});
+
+test("formatAnimatedCount uses stable grouped ASCII digits", () => {
+  assert.equal(formatAnimatedCount(1234), "1,234");
+});
+
+test("getAnimatedCountSlots right-aligns digits for rollover", () => {
+  assert.deepEqual(getAnimatedCountSlots("9", "10"), [
+    { current: "1", isDigit: true, place: 1, previous: "" },
+    { current: "0", isDigit: true, place: 0, previous: "9" },
+  ]);
+});
+
+test("getAnimatedCountSlots shrinks to current width after settling", () => {
+  assert.equal(getAnimatedCountSlots("1,000", "999").length, 5);
+  assert.deepEqual(getAnimatedCountSlots("999", "999"), [
+    { current: "9", isDigit: true, place: 2, previous: "9" },
+    { current: "9", isDigit: true, place: 1, previous: "9" },
+    { current: "9", isDigit: true, place: 0, previous: "9" },
+  ]);
+});

desktop/src/shared/ui/animatedCountParts.ts

@@ -0,0 +1,47 @@
+export type AnimatedCountSlot = {
+  current: string;
+  isDigit: boolean;
+  place: number;
+  previous: string;
+};
+
+const COUNT_FORMATTER = new Intl.NumberFormat("en-US", {
+  maximumFractionDigits: 0,
+});
+
+export function normalizeAnimatedCount(value: number): number {
+  if (!Number.isFinite(value)) return 0;
+  return Math.max(0, Math.trunc(value));
+}
+
+export function formatAnimatedCount(value: number): string {
+  return COUNT_FORMATTER.format(normalizeAnimatedCount(value));
+}
+
+export function isAsciiDigit(value: string): boolean {
+  return value.length === 1 && value >= "0" && value <= "9";
+}
+
+export function getAnimatedCountSlots(
+  previous: string,
+  current: string,
+): AnimatedCountSlot[] {
+  const width = Math.max(previous.length, current.length);
+  const previousOffset = width - previous.length;
+  const currentOffset = width - current.length;
+
+  return Array.from({ length: width }, (_, index) => {
+    const previousCharacter =
+      index >= previousOffset ? previous[index - previousOffset] : "";
+    const currentCharacter =
+      index >= currentOffset ? current[index - currentOffset] : "";
+
+    return {
+      current: currentCharacter ?? "",
+      isDigit:
+        isAsciiDigit(previousCharacter) || isAsciiDigit(currentCharacter),
+      place: width - index - 1,
+      previous: previousCharacter ?? "",
+    };
+  });
+}