fix(ui): centre the quick-tools bar in the margin beside the page (#5635)

crates/ui-egui/src/canvas.rs

@@ -3486,7 +3486,9 @@ fn apply_frame_requests(app: &mut PdfCraftApp, index: usize, ui: &mut egui::Ui,
     // Read mode hides the tools (#2646), and full screen is the page and nothing else (#1459);
     // the quick tools come back with the window.
     if app.mode != crate::Mode::Read && !app.full_screen {
-        quick_bar(app, avail, ui);
+        // The page nearest the left edge, whose margin the bar sits in.
+        let page_left = app.views[index].screen_rects.iter().map(|(_, r)| r.left()).reduce(f32::min);
+        quick_bar(app, avail, page_left, ui);
     }
     if let Some((source, file, destination)) = remote_open {
         app.open_remote_link(source, file, destination);

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

@@ -276,10 +276,23 @@ fn notice_bar(ui: &mut egui::Ui, t: &Tokens, icon: &str, icon_color: Color32, te
     });
 }
 
-/// The floating quick-action bar at the left edge of the document area.
-pub(super) fn quick_bar(app: &mut PdfCraftApp, area: Rect, ui: &mut egui::Ui) {
+const QUICK_BAR_WIDTH: f32 = 46.0;
+const QUICK_BAR_EDGE: f32 = 14.0;
+
+/// How far from the side panel the quick-action bar stands, given the width of the margin between
+/// the panel and the page: centred in it when it holds the bar, else 14 pt in (a page filling the
+/// width).
+fn quick_bar_offset(margin: f32) -> f32 {
+    // shortcut: centred in at most 100 pt of margin, so a wide margin keeps the bar by the panel.
+    let margin = margin.min(100.0);
+    if margin >= QUICK_BAR_WIDTH + 2.0 * 8.0 { (margin - QUICK_BAR_WIDTH) / 2.0 } else { QUICK_BAR_EDGE }
+}
+
+/// The floating quick-action bar in the margin between the side panel and the page.
+pub(super) fn quick_bar(app: &mut PdfCraftApp, area: Rect, page_left: Option<f32>, ui: &mut egui::Ui) {
     let t = Tokens::get(ui.ctx());
-    let pos = area.left_top() + vec2(14.0, 14.0);
+    let x = quick_bar_offset(page_left.map_or(0.0, |left| left - area.left()));
+    let pos = area.left_top() + vec2(x, QUICK_BAR_EDGE);
     egui::Area::new(egui::Id::unique("quick-bar")).order(egui::Order::Middle).fixed_pos(pos).show(ui.ctx(), |ui| {
         egui::Frame::NONE
             .fill(t.card)
@@ -496,3 +509,23 @@ pub(super) fn quick_bar(app: &mut PdfCraftApp, area: Rect, ui: &mut egui::Ui) {
             });
     });
 }
+
+#[cfg(test)]
+mod tests {
+    use super::quick_bar_offset;
+
+    #[test]
+    fn the_quick_bar_centres_in_the_margin_that_holds_it() {
+        assert_eq!(quick_bar_offset(72.0), 13.0);
+        assert_eq!(quick_bar_offset(62.0), 8.0);
+        // A wide margin centres in its first 100 pt, not all of it.
+        assert_eq!(quick_bar_offset(400.0), 27.0);
+    }
+
+    #[test]
+    fn the_quick_bar_keeps_14_pt_in_when_the_margin_cannot_hold_it() {
+        for margin in [61.9, 30.0, 0.0, -50.0, f32::NEG_INFINITY] {
+            assert_eq!(quick_bar_offset(margin), 14.0, "{margin}");
+        }
+    }
+}