fix(ui): add accelerated scrolling on alternate buffer mode (#23940)

Co-authored-by: jacob314 <jacob314@gmail.com>

packages/cli/src/ui/contexts/ScrollProvider.test.tsx

@@ -14,6 +14,13 @@ import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
 import { useRef, useImperativeHandle, forwardRef, type RefObject } from 'react';
 import { Box, type DOMElement } from 'ink';
 import type { MouseEvent } from '../hooks/useMouse.js';
+import { terminalCapabilityManager } from '../utils/terminalCapabilityManager.js';
+
+vi.mock('../utils/terminalCapabilityManager.js', () => ({
+  terminalCapabilityManager: {
+    isGhosttyTerminal: vi.fn(() => false),
+  },
+}));
 
 // Mock useMouse hook
 const mockUseMouseCallbacks = new Set<(event: MouseEvent) => void | boolean>();
@@ -78,6 +85,7 @@ describe('ScrollProvider', () => {
   });
 
   afterEach(() => {
+    vi.restoreAllMocks();
     vi.useRealTimers();
   });
 
@@ -520,4 +528,157 @@ describe('ScrollProvider', () => {
 
     expect(scrollBy).toHaveBeenCalled();
   });
+
+  describe('Scroll Acceleration', () => {
+    it('accelerates scroll for non-Ghostty terminals during rapid scrolling', async () => {
+      const scrollBy = vi.fn();
+      const getScrollState = vi.fn(() => ({
+        scrollTop: 50,
+        scrollHeight: 1000,
+        innerHeight: 10,
+      }));
+
+      vi.mocked(terminalCapabilityManager.isGhosttyTerminal).mockReturnValue(
+        false,
+      );
+
+      await render(
+        <ScrollProvider>
+          <TestScrollable
+            id="test-scrollable"
+            scrollBy={scrollBy}
+            getScrollState={getScrollState}
+          />
+        </ScrollProvider>,
+      );
+
+      const mouseEvent: MouseEvent = {
+        name: 'scroll-down',
+        col: 5,
+        row: 5,
+        shift: false,
+        ctrl: false,
+        meta: false,
+        button: 'none',
+      };
+
+      // Perform 60 rapid scrolls (within 50ms of each other)
+      for (let i = 0; i < 60; i++) {
+        for (const callback of mockUseMouseCallbacks) {
+          callback(mouseEvent);
+        }
+        // Advance time by 10ms for each scroll
+        vi.advanceTimersByTime(10);
+      }
+
+      await vi.runAllTimersAsync();
+
+      // We sum all calls to scrollBy as they might have been flushed individually due to advanceTimersByTime
+      const totalDelta = scrollBy.mock.calls.reduce(
+        (sum, call) => sum + call[0],
+        0,
+      );
+      expect(totalDelta).toBeGreaterThan(60);
+      expect(totalDelta).toBe(150);
+    });
+
+    it('does not accelerate for Ghostty terminals even during rapid scrolling', async () => {
+      const scrollBy = vi.fn();
+      const getScrollState = vi.fn(() => ({
+        scrollTop: 50,
+        scrollHeight: 1000,
+        innerHeight: 10,
+      }));
+
+      vi.mocked(terminalCapabilityManager.isGhosttyTerminal).mockReturnValue(
+        true,
+      );
+
+      await render(
+        <ScrollProvider>
+          <TestScrollable
+            id="test-scrollable"
+            scrollBy={scrollBy}
+            getScrollState={getScrollState}
+          />
+        </ScrollProvider>,
+      );
+
+      const mouseEvent: MouseEvent = {
+        name: 'scroll-down',
+        col: 5,
+        row: 5,
+        shift: false,
+        ctrl: false,
+        meta: false,
+        button: 'none',
+      };
+
+      for (let i = 0; i < 60; i++) {
+        for (const callback of mockUseMouseCallbacks) {
+          callback(mouseEvent);
+        }
+        vi.advanceTimersByTime(10);
+      }
+
+      await vi.runAllTimersAsync();
+
+      // No acceleration means 60 scrolls = delta 60
+      const totalDelta = scrollBy.mock.calls.reduce(
+        (sum, call) => sum + call[0],
+        0,
+      );
+      expect(totalDelta).toBe(60);
+    });
+
+    it('resets acceleration count if scrolling is slow', async () => {
+      const scrollBy = vi.fn();
+      const getScrollState = vi.fn(() => ({
+        scrollTop: 50,
+        scrollHeight: 1000,
+        innerHeight: 10,
+      }));
+
+      vi.mocked(terminalCapabilityManager.isGhosttyTerminal).mockReturnValue(
+        false,
+      );
+
+      await render(
+        <ScrollProvider>
+          <TestScrollable
+            id="test-scrollable"
+            scrollBy={scrollBy}
+            getScrollState={getScrollState}
+          />
+        </ScrollProvider>,
+      );
+
+      const mouseEvent: MouseEvent = {
+        name: 'scroll-down',
+        col: 5,
+        row: 5,
+        shift: false,
+        ctrl: false,
+        meta: false,
+        button: 'none',
+      };
+
+      // Perform scrolls with 100ms gap (greater than 50ms threshold)
+      for (let i = 0; i < 60; i++) {
+        for (const callback of mockUseMouseCallbacks) {
+          callback(mouseEvent);
+        }
+        vi.advanceTimersByTime(100);
+      }
+
+      await vi.runAllTimersAsync();
+
+      // No acceleration because gaps were too large
+      const totalDelta = scrollBy.mock.calls.reduce(
+        (sum, call) => sum + call[0],
+        0,
+      );
+      expect(totalDelta).toBe(60);
+    });
+  });
 });

packages/cli/src/ui/contexts/ScrollProvider.tsx

@@ -16,6 +16,7 @@ import {
 } from 'react';
 import { getBoundingBox, type DOMElement } from 'ink';
 import { useMouse, type MouseEvent } from '../hooks/useMouse.js';
+import { terminalCapabilityManager } from '../utils/terminalCapabilityManager.js';
 
 export interface ScrollState {
   scrollTop: number;
@@ -125,8 +126,33 @@ export const ScrollProvider: React.FC<{ children: React.ReactNode }> = ({
     }
   }, []);
 
+  const scrollMomentumRef = useRef({
+    count: 0,
+    lastTime: 0,
+  });
+
   const handleScroll = (direction: 'up' | 'down', mouseEvent: MouseEvent) => {
-    const delta = direction === 'up' ? -1 : 1;
+    let multiplier = 1;
+    const now = Date.now();
+
+    if (!terminalCapabilityManager.isGhosttyTerminal()) {
+      const timeSinceLastScroll = now - scrollMomentumRef.current.lastTime;
+      // 50ms threshold to consider scrolls consecutive
+      if (timeSinceLastScroll < 50) {
+        scrollMomentumRef.current.count += 1;
+        // Accelerate up to 3x, starting after 5 consecutive scrolls.
+        // Each consecutive scroll increases the multiplier by 0.1.
+        multiplier = Math.min(
+          3,
+          1 + Math.max(0, scrollMomentumRef.current.count - 5) * 0.1,
+        );
+      } else {
+        scrollMomentumRef.current.count = 0;
+      }
+    }
+    scrollMomentumRef.current.lastTime = now;
+
+    const delta = (direction === 'up' ? -1 : 1) * multiplier;
     const candidates = findScrollableCandidates(
       mouseEvent,
       scrollablesRef.current,
@@ -142,15 +168,16 @@ export const ScrollProvider: React.FC<{ children: React.ReactNode }> = ({
       const canScrollUp = effectiveScrollTop > 0.001;
       const canScrollDown =
         effectiveScrollTop < scrollHeight - innerHeight - 0.001;
+      const totalDelta = Math.round(pendingDelta + delta);
 
       if (direction === 'up' && canScrollUp) {
-        pendingScrollsRef.current.set(candidate.id, pendingDelta + delta);
+        pendingScrollsRef.current.set(candidate.id, totalDelta);
         scheduleFlush();
         return true;
       }
 
       if (direction === 'down' && canScrollDown) {
-        pendingScrollsRef.current.set(candidate.id, pendingDelta + delta);
+        pendingScrollsRef.current.set(candidate.id, totalDelta);
         scheduleFlush();
         return true;
       }