From 3539a72b2f30e7f42031a65452d4ad69473d6463 Mon Sep 17 00:00:00 2001
From: noah1400 <48360809+noah1400@users.noreply.github.com>
Date: Tue, 13 Oct 2026 22:23:48 +0200
Subject: [PATCH] Canvas: baseline grid draws only the lines on the page (#844)

The grid was walked from its start, one line per step, with no
bound. A start far above the page (from document.preferences, a
saved file or an IDML package) made the next frame draw lines until
the app ran out of memory. The lines are now the ones inside the
page, found without walking those above it, and never more than one
per point of the tallest page.
---
 crates/ui-egui/src/canvas.rs | 29 ++++++++++++++++++++++++++---
 1 file changed, 26 insertions(+), 3 deletions(-)

diff --git a/crates/ui-egui/src/canvas.rs b/crates/ui-egui/src/canvas.rs
index 077ae2944..45cfbbe68 100644
--- a/crates/ui-egui/src/canvas.rs
+++ b/crates/ui-egui/src/canvas.rs
@@ -584,6 +584,17 @@ fn dashed(painter: &egui::Painter, a: Pos2, b: Pos2, stroke: Stroke, dash: f32,
     painter.extend(egui::Shape::dashed_line(&[a, b], stroke, dash, gap));
 }
 
+/// The most baseline grid lines drawn on a page: one per point of the tallest page (216 in).
+const MAX_BASELINES: usize = 216 * 72;
+
+/// Baseline grid lines inside a page, measured from its top: `start + n * increment` for n >= 0 (the bottom
+/// edge is not a line). Lines above the page are skipped without walking them, however far up the start is.
+fn baseline_grid_lines(start: f64, increment: f64, height: f64) -> impl Iterator<Item = f64> {
+    let step = increment.max(1.0);
+    let first = if start < 0.0 { start.rem_euclid(step) } else { start };
+    (0..MAX_BASELINES).map(move |n| first + n as f64 * step).take_while(move |y| *y < height)
+}
+
 fn draw_guides(app: &DesignApp, painter: &egui::Painter, xf: &Xf, doc: &Document, layout: &CanvasLayout, t: &Tokens) {
     let s = &doc.settings;
     for slot in &layout.slots {
@@ -612,13 +623,11 @@ fn draw_guides(app: &DesignApp, painter: &egui::Painter, xf: &Xf, doc: &Document
             // Baseline grid.
             if app.ui.baseline_grid && xf.zoom >= s.baseline_grid.view_threshold {
                 let g = &s.baseline_grid;
-                let mut y = g.start;
-                while y < p.height {
+                for y in baseline_grid_lines(g.start, g.increment, p.height) {
                     painter.line_segment(
                         [pt(Point::new(pr.x0, pr.y0 + y)), pt(Point::new(pr.x1, pr.y0 + y))],
                         Stroke::new(hair(painter), c32(g.color).gamma_multiply(0.8)),
                     );
-                    y += g.increment.max(1.0);
                 }
             }
             // Margins (magenta) and columns (violet).
@@ -1918,4 +1927,18 @@ mod tests {
         right_click(&mut h, pos2(rect.center().x, rect.min.y - RULER / 2.0));
         assert!(h.query_by_label("   Points").is_none());
     }
+
+    #[test]
+    fn baseline_grid_lines_are_the_ones_on_the_page() {
+        // A start far above the page used to be walked line by line, without end.
+        for start in [-1e9, -1e17] {
+            let mut far = baseline_grid_lines(start, 12.0, 792.0);
+            assert_eq!((far.next(), far.count()), (Some(8.0), 65));
+        }
+        let lines = |start, increment, height| baseline_grid_lines(start, increment, height).collect::<Vec<_>>();
+        assert_eq!(lines(36.0, 12.0, 72.0), [36.0, 48.0, 60.0], "the bottom edge is not a line");
+        assert_eq!(lines(-30.0, 12.0, 30.0), [6.0, 18.0], "a start above the page keeps its rhythm");
+        assert_eq!(lines(f64::NAN, 12.0, 792.0), [0.0; 0]);
+        assert_eq!(baseline_grid_lines(0.0, 12.0, f64::INFINITY).count(), MAX_BASELINES);
+    }
 }