From bed9a48ccd68f40732ca21c71a7a51dd489c2d03 Mon Sep 17 00:00:00 2001
From: Omar Elders <118695915+omarelders@users.noreply.github.com>
Date: Sun, 11 Oct 2026 20:32:38 +0300
Subject: [PATCH] fix(ui): preserve scroll position when collapsing or resizing
 sidebars (#1056) (#1069)

* fix(ui): preserve scroll position when collapsing or resizing sidebars (#1056)

* Run cargo fmt

---------

Co-authored-by: Brandon Thomas <echelon@gmail.com>
---
 crates/ui-egui/src/canvas.rs | 20 ++++++++++++
 crates/ui-egui/tests/view.rs | 63 ++++++++++++++++++++++++++++++++++++
 2 files changed, 83 insertions(+)

diff --git a/crates/ui-egui/src/canvas.rs b/crates/ui-egui/src/canvas.rs
index f5e1b00e..7aeb0c20 100644
--- a/crates/ui-egui/src/canvas.rs
+++ b/crates/ui-egui/src/canvas.rs
@@ -1803,7 +1803,27 @@ pub fn document_area(app: &mut PdfCraftApp, index: usize, ui: &mut egui::Ui) {
     let avail = ui.available_rect_before_wrap();
     view.viewport_w = avail.width();
     view.viewport_h = avail.height();
+    let old_zoom = view.zoom;
     view.fit_zoom(info);
+    if view.shown && view.zoom_anchor.is_none() && view.goto.is_none() && view.goto_point.is_none() && (view.zoom - old_zoom).abs() > 1e-4 {
+        let prev_viewport = view.viewport_screen;
+        let top_y = prev_viewport.min.y;
+        let anchor = view
+            .screen_rects
+            .iter()
+            .find(|(_, r)| r.min.y <= top_y && r.max.y > top_y)
+            .map(|(p, r)| {
+                let fy = if r.height() > 0.0 { ((top_y - r.min.y) / r.height()).clamp(0.0, 1.0) } else { 0.0 };
+                (*p, fy, 0.0f32)
+            })
+            .or_else(|| view.screen_rects.iter().find(|(_, r)| r.min.y >= top_y).map(|(p, r)| (*p, 0.0f32, (r.min.y - top_y).max(0.0))))
+            .or_else(|| view.screen_rects.last().map(|(p, r)| (*p, 1.0f32, (r.max.y - top_y).max(0.0))));
+        if let Some((page, fy, rel_y)) = anchor {
+            view.zoom_anchor = Some((page, 0.5, fy, vec2(avail.width() / 2.0, rel_y)));
+        } else {
+            view.zoom_anchor = Some((view.current, 0.5, 0.0, vec2(avail.width() / 2.0, MARGIN)));
+        }
+    }
 
     // Pinch / ctrl+scroll zoom, anchored on the current page.
     let zoom_delta = ui.input(|i| i.zoom_delta());
diff --git a/crates/ui-egui/tests/view.rs b/crates/ui-egui/tests/view.rs
index bc216c23..12c7e904 100644
--- a/crates/ui-egui/tests/view.rs
+++ b/crates/ui-egui/tests/view.rs
@@ -920,3 +920,66 @@ trailer << /Root 1 0 R >>
     let first = detail(&mut h);
     assert!(first * 10 > sharp * 7, "the first frame after zooming lost its detail: {first} against {sharp}");
 }
+
+#[test]
+fn sidebar_collapse_and_resize_preserves_scroll_position() {
+    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("pages.pdf", None, PAGES.to_vec()).expect("opens");
+        app.set_option("left", "closed").unwrap();
+        app.set_option("panel", "bookmarks").unwrap();
+        app.set_option("layout", "continuous").unwrap();
+        app
+    });
+    let mut width = 0.0;
+    for _ in 0..400 {
+        h.run_steps(1);
+        let w = h.state().views[0].viewport_rect().width();
+        if w == width {
+            break;
+        }
+        width = w;
+    }
+    h.state_mut().views[0].go_to_page(2);
+    h.run_steps(6);
+
+    let v = &h.state().views[0];
+    assert_eq!(v.current, 2);
+    assert_eq!(v.fit, Fit::Width);
+    let zoom_before = v.zoom;
+    let rect_before = v.page_screen_rect(2).expect("page 2 on screen");
+    let vp_before = v.viewport_rect();
+    let rel_y_before = rect_before.min.y - vp_before.min.y;
+
+    // Collapse bookmarks panel: available width increases, so fit-width zoom increases.
+    h.state_mut().set_option("panel", "none").unwrap();
+    h.run_steps(6);
+
+    let v = &h.state().views[0];
+    let zoom_after = v.zoom;
+    assert!(zoom_after > zoom_before, "zoom increased when panel collapsed: {zoom_after} vs {zoom_before}");
+    assert_eq!(v.current, 2, "still on page 2 after panel collapsed");
+    let rect_after = v.page_screen_rect(2).expect("page 2 still on screen");
+    let vp_after = v.viewport_rect();
+    let rel_y_after = rect_after.min.y - vp_after.min.y;
+    assert!((rel_y_after - rel_y_before).abs() < 1.0, "page 2 scroll position preserved: before {rel_y_before}, after {rel_y_after}");
+
+    // Reopen bookmarks panel: available width decreases, so fit-width zoom decreases.
+    h.state_mut().set_option("panel", "bookmarks").unwrap();
+    for _ in 0..400 {
+        h.run_steps(1);
+        let w = h.state().views[0].viewport_rect().width();
+        if w == width {
+            break;
+        }
+        width = w;
+    }
+    let v = &h.state().views[0];
+    assert_eq!(v.current, 2, "still on page 2 after panel reopened");
+    let rect_reopened = v.page_screen_rect(2).expect("page 2 still on screen");
+    let vp_reopened = v.viewport_rect();
+    let rel_y_reopened = rect_reopened.min.y - vp_reopened.min.y;
+    assert!((rel_y_reopened - rel_y_before).abs() < 1.0, "page 2 scroll position preserved on reopen: before {rel_y_before}, after {rel_y_reopened}");
+}