fix(editor): PageDown advances on consecutive long wrapped lines

[JohnMcLear]

Summary

Fix for #4562 — PageDown (and PageUp) became no-ops when the cursor sat on a very long wrapped line and the following lines were also very long.

Root cause

The page up/down handler in ace2_inner.ts advances the caret by numberOfLinesInViewport, computed from scroll.getVisibleLineRange(rep). That helper returns indices into rep.lines — logical lines, not visual/wrapped rows. When a single wrapped logical line fills the viewport (e.g., three ~2000-char lines), the range collapses to [n, n] and the advance count becomes 0:

// before
const numberOfLinesInViewport = newVisibleLineRange[1] - newVisibleLineRange[0];
// then: rep.selStart[0] += 0  →  caret doesn't move
//        scroll.setScrollY(caretOffsetTop)  →  scroll doesn't move

Fix: clamp to at least one logical line so the caret and viewport always advance.

Test plan

• Playwright regression test reproducing the reporter's scenario (three ~2000-char lines, Ctrl+Home, PageDown → viewport scrolls or caret advances)
• Existing page_up_down.spec.ts still passes (no behavior change for the common case where the viewport spans multiple logical lines)
• pnpm run ts-check clean locally

Closes #4562

🤖 Generated with Claude Code

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Review Summary by Qodo

Fix PageDown/PageUp advance on consecutive long wrapped lines

🐞 Bug fix

Walkthroughs

Description

• Fix PageDown/PageUp becoming no-ops on consecutive long wrapped lines
• Clamp viewport line count to minimum of 1 logical line
• Add regression test for issue #4562 with very long wrapped lines

Diagram

flowchart LR
  A["getVisibleLineRange returns logical line indices"] -->|"collapses to [n,n] for wrapped lines"| B["numberOfLinesInViewport = 0"]
  B -->|"causes no-op"| C["PageDown/PageUp fails"]
  D["Math.max clamp to 1"] -->|"guarantees minimum advance"| E["PageDown/PageUp always moves"]

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File Changes

1. src/static/js/ace2_inner.ts 🐞 Bug fix +7/-2

Clamp viewport line count to minimum of 1

• Changed numberOfLinesInViewport calculation to use Math.max(1, ...) to guarantee at least one
 line of movement
• Enhanced comment explaining that getVisibleLineRange returns logical line indices, not visual
 rows
• Prevents PageDown/PageUp from becoming no-ops when a single wrapped line fills the viewport

src/static/js/ace2_inner.ts

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2. src/tests/frontend-new/specs/pagedown_wrapped_lines.spec.ts 🧪 Tests +77/-0

Add regression test for wrapped lines PageDown

• New Playwright regression test for issue #4562
• Tests PageDown behavior with three consecutive ~2000-character lines that wrap extensively
• Verifies that either viewport scrolls or caret advances to a later logical line
• Validates fix prevents the pre-fix behavior where both scroll and caret stayed put

src/tests/frontend-new/specs/pagedown_wrapped_lines.spec.ts

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