Issue #189: each document tab keeps its own scroll position (#259)

The page view's ScrollArea had no id of its own, so egui derived it from
the parent Ui, which is the same for every document. All tabs therefore
shared one scroll offset: switching to another document showed it at
wherever the previous one had been scrolled to (clamped to its length),
and switching back showed the first document at the other one's offset,
usually the top.

Salt the ScrollArea with the document's id so egui keeps one offset per
tab.

Test: viewer.rs::each_document_keeps_its_own_scroll_position scrolls
a.pdf to page 4, switches to b.pdf and scrolls it to page 2, then
switches back and forth: before the fix b.pdf opened on page 2 instead
of its own top. Cited as evidence on view.document-tabs.

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

@@ -1174,6 +1174,8 @@ pub fn document_area(app: &mut PdfCraftApp, index: usize, ui: &mut egui::Ui) {
         },
         ..Default::default()
     });
+    // Each document keeps its own scroll position when switching tabs (#189).
+    scroll = scroll.id_salt(("page-view", view.id));
     if let Some((page, fx, fy, rel)) = view.zoom_anchor.take() {
         let r = rects[page.min(rects.len() - 1)];
         let point = pos2(r.left() + fx * r.width(), r.top() - y_shift + fy * r.height());

crates/ui-egui/tests/viewer.rs

@@ -1,5 +1,5 @@
 //! Viewer conveniences: close all, revert, fit height, view history, select all, find options,
-//! cover page.
+//! cover page, a scroll position per tab.
 
 use egui_kittest::Harness;
 use egui_kittest::kittest::Queryable;
@@ -395,3 +395,31 @@ fn optimizer_audits_space_usage() {
     h.run_steps(2);
     assert_eq!(h.state().dialog, Some(Dialog::Optimize), "back to the optimizer");
 }
+
+#[test]
+fn each_document_keeps_its_own_scroll_position() {
+    // #189: every document's page view shared one scroll position, so switching tabs showed a
+    // document at wherever the other one had been scrolled to (usually back at the top).
+    use pdfcraft_ui_egui::canvas::Fit;
+    let mut h = harness();
+    for v in &mut h.state_mut().views {
+        v.fit = Fit::Width;
+    }
+    let show = |h: &mut Harness<'static, PdfCraftApp>, i: usize| {
+        h.state_mut().active = Some(i);
+        h.run_steps(4);
+        h.state().views[i].current
+    };
+    show(&mut h, 0);
+    h.state_mut().views[0].goto = Some((3, 0.0));
+    h.run_steps(4);
+    assert_eq!(h.state().views[0].current, 3, "a.pdf scrolled to page 4");
+    // b.pdf opens where it was (the top), and is scrolled to its own last page.
+    assert_eq!(show(&mut h, 1), 0);
+    h.state_mut().views[1].goto = Some((1, 0.0));
+    h.run_steps(4);
+    assert_eq!(h.state().views[1].current, 1);
+    // Back to a.pdf: still on page 4, and b.pdf still on page 2.
+    assert_eq!(show(&mut h, 0), 3);
+    assert_eq!(show(&mut h, 1), 1);
+}

parity/acrobat-features.toml

@@ -1166,8 +1166,8 @@ milestone = "M3"
 status = "shipped"
 commands = []
 tools = ["doc_list"]
-evidence = ["crates/ui-egui/tests/editing.rs::combining_files_opens_a_new_unsaved_tab", "crates/ui-egui/tests/control.rs::state_and_view_options"]
-notes = ""
+evidence = ["crates/ui-egui/tests/editing.rs::combining_files_opens_a_new_unsaved_tab", "crates/ui-egui/tests/control.rs::state_and_view_options", "crates/ui-egui/tests/viewer.rs::each_document_keeps_its_own_scroll_position"]
+notes = "each tab keeps its own scroll position (#189)"
 
 [[feature]]
 id = "view.multiple-windows"