From 6fa0c3af0c1026e75610a67e2f7bf9d4d49bf549 Mon Sep 17 00:00:00 2001
From: Alejandro Alonso <alejandroalonsofernandez@gmail.com>
Date: Tue, 20 Jan 2026 12:18:18 +0100
Subject: [PATCH] :bug: Fix some tiles disappear after fast zoom and pan

---
 render-wasm/src/main.rs   |  1 +
 render-wasm/src/render.rs |  5 +++++
 render-wasm/src/state.rs  | 10 ++++++++++
 3 files changed, 16 insertions(+)

diff --git a/render-wasm/src/main.rs b/render-wasm/src/main.rs
index 9d4ff41617..162732d551 100644
--- a/render-wasm/src/main.rs
+++ b/render-wasm/src/main.rs
@@ -284,6 +284,7 @@ pub extern "C" fn set_view_end() {
             performance::end_measure!("set_view_end::clear_tile_index");
             performance::end_timed_log!("clear_tile_index", _clear_start);
         }
+        state.render_state.sync_cached_viewbox();
         performance::end_measure!("set_view_end");
         performance::end_timed_log!("set_view_end", _end_start);
         #[cfg(feature = "profile-macros")]
diff --git a/render-wasm/src/render.rs b/render-wasm/src/render.rs
index 1d80780bb1..f0619a9e37 100644
--- a/render-wasm/src/render.rs
+++ b/render-wasm/src/render.rs
@@ -1136,6 +1136,7 @@ impl RenderState {
     ) -> Result<(), String> {
         let _start = performance::begin_timed_log!("start_render_loop");
         let scale = self.get_scale();
+
         self.tile_viewbox.update(self.viewbox, scale);
 
         self.focus_mode.reset();
@@ -2292,6 +2293,10 @@ impl RenderState {
         (self.viewbox.zoom - self.cached_viewbox.zoom).abs() > f32::EPSILON
     }
 
+    pub fn sync_cached_viewbox(&mut self) {
+        self.cached_viewbox = self.viewbox;
+    }
+
     pub fn mark_touched(&mut self, uuid: Uuid) {
         self.touched_ids.insert(uuid);
     }
diff --git a/render-wasm/src/state.rs b/render-wasm/src/state.rs
index 40b272007c..f178ed4477 100644
--- a/render-wasm/src/state.rs
+++ b/render-wasm/src/state.rs
@@ -100,6 +100,16 @@ impl<'a> State<'a> {
     }
 
     pub fn start_render_loop(&mut self, timestamp: i32) -> Result<(), String> {
+        // If zoom changed, we MUST rebuild the tile index before using it.
+        // Otherwise, the index will have tiles from the old zoom level, causing visible
+        // tiles to appear empty. This can happen if start_render_loop() is called before
+        // set_view_end() finishes rebuilding the index, or if set_view_end() hasn't been
+        // called yet.
+        let zoom_changed = self.render_state.zoom_changed();
+        if zoom_changed {
+            self.rebuild_tiles_shallow();
+        }
+
         self.render_state
             .start_render_loop(None, &self.shapes, timestamp, false)?;
         Ok(())