From b67151fafd00f1f0f5ff844b82ba2d69f3050d5c Mon Sep 17 00:00:00 2001
From: Rob Jacobson <rljacobson@outlook.com>
Date: Mon, 12 Oct 2026 20:10:43 -0700
Subject: [PATCH] fix(android): fit camera to zero-height bounds

- Expand zero-height camera-fitting bounds symmetrically by one millionth of a degree.
- Preserve exact camera constraints and nonzero-height bounds.
- Add regression coverage for the midpoint, unchanged bounds, and latitude limits.
---
 .../rnmbx/components/camera/CameraStop.kt     |  2 +-
 .../com/rnmapbox/rnmbx/utils/LatLngBounds.kt  | 18 +++++++-
 .../rnmapbox/rnmbx/utils/LatLngBoundsTest.kt  | 42 +++++++++++++++++++
 3 files changed, 59 insertions(+), 3 deletions(-)
 create mode 100644 android/src/test/kotlin/com/rnmapbox/rnmbx/utils/LatLngBoundsTest.kt

diff --git a/android/src/main/java/com/rnmapbox/rnmbx/components/camera/CameraStop.kt b/android/src/main/java/com/rnmapbox/rnmbx/components/camera/CameraStop.kt
index 5e6a233852..7e1078bdac 100644
--- a/android/src/main/java/com/rnmapbox/rnmbx/components/camera/CameraStop.kt
+++ b/android/src/main/java/com/rnmapbox/rnmbx/components/camera/CameraStop.kt
@@ -130,7 +130,7 @@ class CameraStop {
             // cameraForCoordinateBounds insets the bounds within the map's current padding, so omitted edges must add nothing.
             val boundsPadding = intArrayOf(mPaddingLeft ?: 0, mPaddingTop ?: 0, mPaddingRight ?: 0, mPaddingBottom ?: 0)
             val boundsCamera = map.cameraForCoordinateBounds(
-                mBounds!!.toBounds(),
+                mBounds!!.toCameraBounds(),
                 convert(clippedPadding(boundsPadding, mapView)),
                 bearing,
                 tilt
diff --git a/android/src/main/java/com/rnmapbox/rnmbx/utils/LatLngBounds.kt b/android/src/main/java/com/rnmapbox/rnmbx/utils/LatLngBounds.kt
index 08f612b51b..dca85a2fb3 100644
--- a/android/src/main/java/com/rnmapbox/rnmbx/utils/LatLngBounds.kt
+++ b/android/src/main/java/com/rnmapbox/rnmbx/utils/LatLngBounds.kt
@@ -2,7 +2,6 @@ package com.rnmapbox.rnmbx.utils
 
 import com.mapbox.geojson.Point
 import com.mapbox.maps.CoordinateBounds
-import com.rnmapbox.rnmbx.utils.LatLngBounds
 
 class LatLngBounds internal constructor(
     var latNorth: Double,
@@ -27,9 +26,24 @@ class LatLngBounds internal constructor(
         )
     }
 
+    fun toCameraBounds(): CoordinateBounds {
+        if (latNorth != latSouth) {
+            return toBounds()
+        }
+
+        // Mapbox returns zoom 0 when fitting zero-height bounds (#4050).
+        // Expand only the fitting bounds, preserving the center and exact camera constraints.
+        val latitudePadding = 0.000001
+        return CoordinateBounds(
+            Point.fromLngLat(lonWest, (latSouth - latitudePadding).coerceAtLeast(-90.0)),
+            Point.fromLngLat(lonEast, (latNorth + latitudePadding).coerceAtMost(90.0)),
+            false
+        )
+    }
+
     companion object {
         fun from(bbox: Double, bbox1: Double, bbox2: Double, bbox3: Double): LatLngBounds {
             return LatLngBounds(bbox, bbox1, bbox2, bbox3)
         }
     }
-}
\ No newline at end of file
+}
diff --git a/android/src/test/kotlin/com/rnmapbox/rnmbx/utils/LatLngBoundsTest.kt b/android/src/test/kotlin/com/rnmapbox/rnmbx/utils/LatLngBoundsTest.kt
new file mode 100644
index 0000000000..970f098cb8
--- /dev/null
+++ b/android/src/test/kotlin/com/rnmapbox/rnmbx/utils/LatLngBoundsTest.kt
@@ -0,0 +1,42 @@
+package com.rnmapbox.rnmbx.utils
+
+import org.junit.jupiter.api.Assertions.*
+import org.junit.jupiter.api.Test
+
+class LatLngBoundsTest {
+    @Test
+    fun `horizontal camera bounds enclose the original line without shifting its center`() {
+        val bounds = LatLngBounds.from(37.0, -121.2, 37.0, -122.1)
+        val cameraBounds = bounds.toCameraBounds()
+
+        assertTrue(cameraBounds.southwest.latitude() < 37.0)
+        assertTrue(cameraBounds.northeast.latitude() > 37.0)
+        assertEquals(37.0, (cameraBounds.southwest.latitude() + cameraBounds.northeast.latitude()) / 2, 1e-12)
+        assertEquals(-122.1, cameraBounds.southwest.longitude())
+        assertEquals(-121.2, cameraBounds.northeast.longitude())
+
+        // Camera constraints must still use the exact bounds after a fit.
+        assertEquals(37.0, bounds.toBounds().southwest.latitude())
+        assertEquals(37.0, bounds.toBounds().northeast.latitude())
+    }
+
+    @Test
+    fun `vertical and very thin nonzero bounds retain their exact coordinates`() {
+        for (bounds in listOf(
+            LatLngBounds.from(37.1, -121.1, 37.0, -121.1),
+            LatLngBounds.from(37.00000001, -121.2, 37.0, -122.1)
+        )) {
+            assertEquals(bounds.toBounds(), bounds.toCameraBounds())
+        }
+    }
+
+    @Test
+    fun `expanding a horizontal fit never exceeds valid latitudes`() {
+        for (latitude in listOf(-90.0, 90.0)) {
+            val cameraBounds = LatLngBounds.from(latitude, 1.0, latitude, -1.0).toCameraBounds()
+            assertTrue(cameraBounds.southwest.latitude() >= -90.0)
+            assertTrue(cameraBounds.northeast.latitude() <= 90.0)
+            assertTrue(cameraBounds.southwest.latitude() < cameraBounds.northeast.latitude())
+        }
+    }
+}