From cba51cfb7e09f3cf3c7f36afe52daab337f6ff41 Mon Sep 17 00:00:00 2001
From: Alejandro Alonso <alejandro.alonso@kaleidos.net>
Date: Tue, 29 Sep 2026 12:38:04 +0200
Subject: [PATCH] :bug: Keep adjacent tile compose origins abutted (#11968)

Round the view offset once and place tiles at
origin + k * TILE_SIZE so half-pixel pans do not
open a 1px background seam between neighbors.
---
 render-wasm/src/render/surfaces.rs | 16 +++---------
 render-wasm/src/tiles.rs           | 39 ++++++++++++++++++++++++++++++
 2 files changed, 43 insertions(+), 12 deletions(-)

diff --git a/render-wasm/src/render/surfaces.rs b/render-wasm/src/render/surfaces.rs
index c15b4fc5db..761b2d02d6 100644
--- a/render-wasm/src/render/surfaces.rs
+++ b/render-wasm/src/render/surfaces.rs
@@ -1852,14 +1852,8 @@ impl TileTextureCache {
                     continue;
                 }
 
-                self.transforms[index] = skia::RSXform::new(
-                    dest_scale,
-                    0.0,
-                    (
-                        (x as f32 * self.tile_size - offset.x).round(),
-                        (y as f32 * self.tile_size - offset.y).round(),
-                    ),
-                );
+                let (tx, ty) = tiles::tile_screen_xy(Tile(x, y), self.tile_size, offset);
+                self.transforms[index] = skia::RSXform::new(dest_scale, 0.0, (tx, ty));
 
                 let src = tiles::tile_atlas_content_rect(tile_ref.rect, self.slot_size);
                 self.textures[index].set_ltrb(src.left, src.top, src.right, src.bottom);
@@ -1903,8 +1897,7 @@ impl TileTextureCache {
 
                 let src = tiles::tile_atlas_content_rect(tile_ref.rect, self.slot_size);
                 let scos = doc_rect.width() * s / src.width();
-                let tx = ((doc_rect.left + viewbox.pan.x) * s).round();
-                let ty = ((doc_rect.top + viewbox.pan.y) * s).round();
+                let (tx, ty) = tiles::doc_rect_screen_xy(doc_rect, viewbox.pan, s);
 
                 transforms.push(skia::RSXform::new(scos, 0.0, (tx, ty)));
                 textures.push(src);
@@ -1932,8 +1925,7 @@ impl TileTextureCache {
             }
 
             let src = tiles::tile_atlas_content_rect(tile_ref.rect, self.slot_size);
-            let tx = ((doc_rect.left + viewbox.pan.x) * s).round();
-            let ty = ((doc_rect.top + viewbox.pan.y) * s).round();
+            let (tx, ty) = tiles::doc_rect_screen_xy(doc_rect, viewbox.pan, s);
             let scos = doc_rect.width() * s / src.width();
 
             transforms.push(skia::RSXform::new(scos, 0.0, (tx, ty)));
diff --git a/render-wasm/src/tiles.rs b/render-wasm/src/tiles.rs
index c4783af4f4..8180266fb9 100644
--- a/render-wasm/src/tiles.rs
+++ b/render-wasm/src/tiles.rs
@@ -211,6 +211,22 @@ impl TileViewbox {
 
 pub const TILE_SIZE: f32 = 512.;
 
+/// Screen-space top-left of `tile`: `round(-offset) + tile * tile_size`.
+pub fn tile_screen_xy(tile: Tile, tile_size: f32, offset: skia::Point) -> (f32, f32) {
+    (
+        (-offset.x).round() + tile.x() as f32 * tile_size,
+        (-offset.y).round() + tile.y() as f32 * tile_size,
+    )
+}
+
+/// Screen-space top-left of `doc_rect`: `doc * scale + round(pan * scale)`.
+pub fn doc_rect_screen_xy(doc_rect: skia::Rect, pan: skia::Point, scale: f32) -> (f32, f32) {
+    (
+        doc_rect.left * scale + (pan.x * scale).round(),
+        doc_rect.top * scale + (pan.y * scale).round(),
+    )
+}
+
 #[inline(always)]
 pub fn get_tile_dimensions() -> skia::ISize {
     (TILE_SIZE as i32, TILE_SIZE as i32).into()
@@ -501,6 +517,29 @@ mod tests {
     use super::*;
     use skia_safe as skia;
 
+    #[test]
+    fn tile_screen_xy_keeps_neighbors_abutted_at_half_pixel_offset() {
+        let offset = skia::Point::new(103.5, 0.0);
+        let (x0, y0) = tile_screen_xy(Tile(0, 0), TILE_SIZE, offset);
+        let (x1, y1) = tile_screen_xy(Tile(1, 0), TILE_SIZE, offset);
+        let (x2, y2) = tile_screen_xy(Tile(0, 1), TILE_SIZE, offset);
+        assert_eq!(x1 - x0, TILE_SIZE);
+        assert_eq!(y2 - y0, TILE_SIZE);
+        assert_eq!(y1, y0);
+        assert_eq!(x2, x0);
+    }
+
+    #[test]
+    fn doc_rect_screen_xy_keeps_neighbors_abutted_at_half_pixel_pan() {
+        let pan = skia::Point::new(-103.5, 0.0);
+        let scale = 1.0_f32;
+        let a = skia::Rect::from_xywh(0.0, 0.0, TILE_SIZE, TILE_SIZE);
+        let b = skia::Rect::from_xywh(TILE_SIZE, 0.0, TILE_SIZE, TILE_SIZE);
+        let (x0, _) = doc_rect_screen_xy(a, pan, scale);
+        let (x1, _) = doc_rect_screen_xy(b, pan, scale);
+        assert_eq!(x1 - x0, TILE_SIZE);
+    }
+
     #[test]
     fn atlas_slot_is_full_size_when_tiles_fit() {
         assert_eq!(tile_atlas_slot_size(64, 4096), 512);