Go to page brings the page into view horizontally too (part of #2665)

crates/ui-egui/src/canvas.rs

@@ -2468,6 +2468,19 @@ pub fn document_area(app: &mut PdfCraftApp, index: usize, ui: &mut egui::Ui) {
         let page = page.min(rects.len() - 1);
         let r = rects[page];
         scroll = scroll.vertical_scroll_offset((r.top() - y_shift - GAP + frac * r.height()).max(0.0));
+        // Horizontally too, as Acrobat does (#2665): beside a much wider page the others sit off
+        // to one side. A page already shown whole, or one filling the window's width, keeps the
+        // scroll, so paging through a zoomed-in document stays on the same column. Otherwise a
+        // page that fits is centred, and a wider one scrolled just far enough to fill the window.
+        let (x, w) = (view.last_scroll.x, avail.width());
+        let target = if r.width() <= w {
+            if r.left() >= x && r.right() <= x + w { x } else { r.center().x - w / 2.0 }
+        } else {
+            x.clamp(r.left(), r.right() - w)
+        };
+        if (target - x).abs() > 0.5 {
+            scroll = scroll.horizontal_scroll_offset(target.clamp(0.0, (content_w - w).max(0.0)));
+        }
     }
     let ppp = ui.ctx().pixels_per_point();
     let scale = view.render_scale(ppp);

crates/ui-egui/tests/viewer.rs

@@ -1738,3 +1738,27 @@ fn arrow_keys_follow_the_scroll_speed_preference() {
     let (slow, normal, fast) = (lines_to_page_2("slow"), lines_to_page_2("normal"), lines_to_page_2("fast"));
     assert!(fast * 3 < normal * 2 && normal * 3 < slow * 2, "presses to page 2: slow {slow}, normal {normal}, fast {fast}");
 }
+
+/// #2665: next to a very wide page the normal pages sit far to its right, outside the window.
+/// Going to one changed the page counter but kept the horizontal scroll, so it stayed out of
+/// sight; as in Acrobat, it is brought into view.
+#[test]
+fn go_to_page_brings_the_page_into_view_horizontally() {
+    use pdfcraft_ui_egui::canvas::Fit;
+    let mut h = Harness::builder().with_size(egui::vec2(1400.0, 900.0)).build_eframe(|_cc| {
+        let mut app = PdfCraftApp::new();
+        app.set_option("language", "en").unwrap();
+        app.open_bytes("wide.pdf", None, sized_fixture(&[(595, 842), (595, 842), (595, 842), (6000, 2000)])).unwrap();
+        app.views[0].fit = Fit::None;
+        app.views[0].zoom = 1.0;
+        app
+    });
+    h.run_steps(4);
+    assert!(h.state().views[0].page_screen_rect(0).is_none(), "page 1 starts off-screen, right of the window");
+    h.state_mut().views[0].go_to_page(2);
+    h.run_steps(4);
+    let v = &h.state().views[0];
+    assert_eq!(v.current, 2);
+    let (r, vp) = (v.page_screen_rect(2).expect("page 3 on screen"), v.viewport_rect());
+    assert!(r.left() >= vp.left() && r.right() <= vp.right(), "page 3 at {r:?}, viewport {vp:?}");
+}