fix(ui): the sliver bookmark_position ignores is one physical pixel, not one point (#5636)

Page rectangles snap to whole physical pixels, so what counts as rounding scales with the display:
the view keeps last frame's pixels per point and the threshold is its inverse.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

crates/ui-egui/src/canvas.rs

@@ -382,6 +382,8 @@ pub struct DocView {
     /// Screen rects of the pages drawn last frame (hit-testing, tests, automation).
     screen_rects: Vec<(usize, Rect)>,
     screen_xforms: Vec<(usize, PageXform)>,
+    /// Pixels per point last frame, for telling a rounding sliver of a page from a visible one.
+    screen_ppp: f32,
     /// The scroll viewport on screen last frame.
     viewport_screen: Rect,
     /// Pending zoom anchor: page, fractional position (also in gutters), and viewport offset.
@@ -669,6 +671,7 @@ impl DocView {
             page_heights: info.pages.iter().map(|p| p.height).collect(),
             screen_rects: Vec::new(),
             screen_xforms: Vec::new(),
+            screen_ppp: 1.0,
             viewport_screen: Rect::NOTHING,
             zoom_anchor: None,
             shown: false,
@@ -2010,6 +2013,7 @@ fn draw_pages(
     let origin = resp_rect.min - vec2(0.0, y_shift);
     let visible = viewport.translate(resp_rect.min.to_vec2());
     view.screen_rects.clear();
+    view.screen_ppp = ui.ctx().pixels_per_point();
     view.screen_xforms.clear();
     // The pointer as the pages see it: nothing over them (a dialog from the frame it opens),
     // or a press they hold. The page modules read it rather than egui's raw pointer state.

crates/ui-egui/src/canvas/navigation.rs

@@ -32,7 +32,7 @@ impl DocView {
         // of pages of different heights, whose shorter page can start below it), else the
         // nearest one below.
         let top = self.viewport_screen.top();
-        // A page showing less than SLIVER is rounding, not what the reader is looking at: a jump
+        // A page showing less than a pixel is rounding, not what the reader is looking at: a jump
         // to the next page leaves the previous one's bottom edge within a pixel of the top. Such a
         // sliver still counts when nothing else shows (the last page at the very end).
         let pick = |slack: f32| {
@@ -42,7 +42,7 @@ impl DocView {
                 .min_by_key(|(p, _)| *p)
                 .or_else(|| showing().min_by(|a, b| a.1.rect.top().total_cmp(&b.1.rect.top()).then(a.0.cmp(&b.0))))
         };
-        let (page, xf) = pick(SLIVER).or_else(|| pick(0.0))?;
+        let (page, xf) = pick(sliver(self.screen_ppp)).or_else(|| pick(0.0))?;
         let p = info.pages.get(*page)?;
         // Above the page (in the gap before it) is the page's own top. The window's vertical is
         // the view's y, or its x when the view is turned a quarter.
@@ -460,6 +460,23 @@ pub fn shortcuts(view: &mut DocView, ctx: &egui::Context, speed: crate::scroll_s
 /// How far ↓ / ↑ scroll at Normal speed (Scroll speed halves or doubles it). Page Down / Page Up
 /// keep this much of the previous screen in view.
 const KEY_SCROLL_LINE: f32 = 40.0;
-/// How much of a page must show at the window's top edge to count as the page there, in points: less
-/// is rounding, since page rectangles snap to whole pixels.
-const SLIVER: f32 = 1.0;
+/// How much of a page must show at the window's top edge to count as the page there, in points: one
+/// physical pixel, since page rectangles snap to whole pixels and less is rounding.
+fn sliver(pixels_per_point: f32) -> f32 {
+    if pixels_per_point.is_finite() && pixels_per_point > 0.0 { 1.0 / pixels_per_point } else { 1.0 }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::sliver;
+
+    #[test]
+    fn a_sliver_is_one_physical_pixel() {
+        assert_eq!(sliver(1.0), 1.0);
+        assert_eq!(sliver(2.0), 0.5);
+        assert_eq!(sliver(0.5), 2.0);
+        for bad in [0.0, -1.0, f32::NAN, f32::INFINITY] {
+            assert_eq!(sliver(bad), 1.0, "{bad}");
+        }
+    }
+}