Issue #185: arrow keys and Page Down / Up move through a scrolling document (#261)

In the continuous (default) and two-page views, → / ← and Page Down /
Page Up only did something with ⌘ held, and ↓ / ↑ never did: egui's
ScrollArea has no keyboard scrolling of its own, so without a modifier
the keyboard could not move through the document at all.

As in Acrobat: → / ← now go to the next / previous page (a spread in
two-page view) in every layout; ↓ / ↑ scroll a line (40 pt, a desktop
mouse-wheel line) and Page Down / Page Up scroll a screen (less one
line) where the pages scroll. ⌘ + those keys and single-page view keep
their page steps. The scroll is queued on the DocView and applied by the
page view's ScrollArea in the same frame, next to the middle-button
auto-scroll.

The shortcuts list shows ← / → beside ⌘← / ⌘→; the view.page-navigation
parity entry cites the new test.

Test: viewer.rs::arrow_and_page_keys_move_through_a_scrolling_document
(continuous at Fit Width: → / ← turn pages; ↓ and Page Down take
several presses to reach page 2, ↓ more than Page Down, and ↑ / Page Up
come back; → in two-page view and Page Down in single-page view). It
failed at the first → before the fix.

Co-authored-by: wstock <1394687+wstock@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

crates/ui-egui/src/canvas.rs

@@ -197,6 +197,9 @@ pub struct DocView {
     pub forward: Vec<usize>,
     /// Pending navigation: page and fraction down the page to align with the viewport top.
     pub goto: Option<(usize, f32)>,
+    /// Pending keyboard scrolling, in points down (negative: up): ↓ / ↑, and Page Down /
+    /// Page Up where the pages scroll.
+    pub key_scroll: f32,
     /// Briefly outline an annotation after navigating to it from a panel.
     pub flash: Option<(usize, [f32; 4], f64)>,
     /// Compare files: differences shaded on this document's pages (page, user-space box, colour).
@@ -331,6 +334,7 @@ impl DocView {
             back: Vec::new(),
             forward: Vec::new(),
             goto: None,
+            key_scroll: 0.0,
             flash: None,
             compare_marks: Vec::new(),
             pages: HashMap::new(),
@@ -1048,18 +1052,36 @@ pub fn shortcuts(view: &mut DocView, ctx: &egui::Context) {
     if key(Key::End) {
         view.go_to_page(usize::MAX);
     }
-    if view.layout == PageLayout::Single || ctx.input(|i| i.modifiers.command) {
-        if key(Key::ArrowRight) || key(Key::PageDown) {
-            view.step_page(true);
-        }
-        if key(Key::ArrowLeft) || key(Key::PageUp) {
-            view.step_page(false);
+    // As in Acrobat: → / ← go to the next / previous page in every layout (#185), and so do
+    // ⌘Page Down / ⌘Page Up, or plain Page Down / Page Up in single-page view.
+    let command = ctx.input(|i| i.modifiers.command);
+    let scrolls = view.layout != PageLayout::Single;
+    if key(Key::ArrowRight) || (key(Key::PageDown) && (command || !scrolls)) {
+        view.step_page(true);
+    }
+    if key(Key::ArrowLeft) || (key(Key::PageUp) && (command || !scrolls)) {
+        view.step_page(false);
+    }
+    // ↓ / ↑ scroll a line, and Page Down / Page Up a screen where the pages scroll.
+    if !command {
+        let screen = (view.viewport_h - KEY_SCROLL_LINE).max(KEY_SCROLL_LINE);
+        let steps = [(Key::ArrowDown, KEY_SCROLL_LINE), (Key::ArrowUp, -KEY_SCROLL_LINE), (Key::PageDown, screen), (Key::PageUp, -screen)];
+        for (k, by) in steps {
+            if key(k) && (scrolls || matches!(k, Key::ArrowDown | Key::ArrowUp)) {
+                view.key_scroll += by;
+            }
         }
     }
 }
 
+/// How far ↓ / ↑ scroll: a mouse-wheel line on the desktop. Page Down / Page Up keep this much
+/// of the previous screen in view.
+const KEY_SCROLL_LINE: f32 = 40.0;
+
 pub fn document_area(app: &mut PdfCraftApp, index: usize, ui: &mut egui::Ui) {
     let t = Tokens::get(ui.ctx());
+    // Keyboard scrolling applies to this frame's page view only, never later.
+    let key_scroll = std::mem::take(&mut app.views[index].key_scroll);
     // The Search panel closed: its search moves to the find bar.
     let search_open = app.right == Some(crate::RightPanel::Search);
     if let Some(f) = app.views[index].find.as_mut()
@@ -1259,8 +1281,10 @@ pub fn document_area(app: &mut PdfCraftApp, index: usize, ui: &mut egui::Ui) {
             // Only interactions pause; the document must keep its original colours.
             ui.set_opacity(opacity);
         }
-        if auto_delta != Vec2::ZERO {
-            ui.scroll_with_delta_animation(auto_delta, egui::style::ScrollAnimation::none());
+        // Middle-button auto-scroll and the keyboard (positive y moves the content down).
+        let delta = auto_delta - vec2(0.0, key_scroll);
+        if delta != Vec2::ZERO {
+            ui.scroll_with_delta_animation(delta, egui::style::ScrollAnimation::none());
         }
         let (resp_rect, resp) = ui.allocate_exact_size(vec2(content_w, content_h), Sense::click_and_drag());
         // The Hand tool pans: the content widget takes every drag, so scroll by its delta.

crates/ui-egui/src/dialogs.rs

@@ -1030,7 +1030,7 @@ pub fn show(app: &mut PdfCraftApp, ctx: &egui::Context) {
                     ("⌘+ / ⌘−", tl!("Zoom in / out (also pinch or ⌘-scroll)")),
                     ("⇧⌘+ / ⇧⌘−", tl!("Rotate view")),
                     ("Home / End", tl!("First / last page")),
-                    ("⌘← / ⌘→", tl!("Previous / next page")),
+                    ("← / →, ⌘← / ⌘→", tl!("Previous / next page")),
                     ("Delete", tl!("Delete selected pages (Organize)")),
                     ("⌘A", tl!("Select all pages (Organize)")),
                 ] {

crates/ui-egui/tests/viewer.rs

@@ -423,3 +423,51 @@ fn each_document_keeps_its_own_scroll_position() {
     assert_eq!(show(&mut h, 0), 3);
     assert_eq!(show(&mut h, 1), 1);
 }
+
+#[test]
+fn arrow_and_page_keys_move_through_a_scrolling_document() {
+    // #185: in the continuous (default) and two-page views the arrow keys and Page Down / Up did
+    // nothing without ⌘.
+    use egui::Key;
+    use pdfcraft_ui_egui::canvas::{Fit, PageLayout};
+    let mut h = harness();
+    h.state_mut().active = Some(0);
+    h.state_mut().views[0].fit = Fit::Width;
+    h.run_steps(4);
+    assert_eq!(h.state().views[0].layout, PageLayout::Continuous);
+    let press = |h: &mut Harness<'static, PdfCraftApp>, key, times: usize| {
+        for _ in 0..times {
+            h.key_press(key);
+            h.run_steps(2);
+        }
+        h.run_steps(2);
+        h.state().views[0].current
+    };
+    // → / ← turn pages, as in Acrobat
+    assert_eq!(press(&mut h, Key::ArrowRight, 2), 2);
+    assert_eq!(press(&mut h, Key::ArrowLeft, 1), 1);
+    assert_eq!(press(&mut h, Key::ArrowLeft, 1), 0);
+    // ↓ scrolls a line at a time, Page Down a screen: count the presses that reach page 2
+    let presses = |h: &mut Harness<'static, PdfCraftApp>, key| {
+        let mut n = 0;
+        while h.state().views[0].current == 0 {
+            press(h, key, 1);
+            n += 1;
+            assert!(n < 500, "{key:?} never reached page 2");
+        }
+        n
+    };
+    let lines = presses(&mut h, Key::ArrowDown);
+    assert_eq!(press(&mut h, Key::ArrowUp, lines + 1), 0);
+    let screens = presses(&mut h, Key::PageDown);
+    assert_eq!(press(&mut h, Key::PageUp, screens + 1), 0);
+    assert!(screens >= 2 && lines > 2 * screens, "a fit-width page: {lines} lines, {screens} screens");
+    // two-page view: → moves a spread
+    h.state_mut().views[0].layout = PageLayout::TwoUp;
+    h.run_steps(4);
+    assert_eq!(press(&mut h, Key::ArrowRight, 1), 2);
+    // single-page view keeps Page Down for the next page
+    h.state_mut().views[0].layout = PageLayout::Single;
+    h.run_steps(4);
+    assert_eq!(press(&mut h, Key::PageDown, 1), 3);
+}

parity/acrobat-features.toml

@@ -1106,8 +1106,8 @@ milestone = "M3"
 status = "shipped"
 commands = []
 tools = []
-evidence = ["crates/ui-egui/tests/control.rs::state_and_view_options", "crates/ui-egui/tests/ui.rs::theme_and_view_options_apply", "crates/ui-egui/tests/viewer.rs::two_page_view_steps_a_spread_at_a_time"]
-notes = "in two-page view, next and previous page move a whole spread (with or without a cover page)"
+evidence = ["crates/ui-egui/tests/control.rs::state_and_view_options", "crates/ui-egui/tests/ui.rs::theme_and_view_options_apply", "crates/ui-egui/tests/viewer.rs::two_page_view_steps_a_spread_at_a_time", "crates/ui-egui/tests/viewer.rs::arrow_and_page_keys_move_through_a_scrolling_document"]
+notes = "in two-page view, next and previous page move a whole spread (with or without a cover page); → / ← turn pages in every layout, ↓ / ↑ scroll a line and Page Down / Page Up a screen (the page in single-page view), as in Acrobat"
 
 [[feature]]
 id = "view.goto-page-label"