Improve particle wind animation: longer lifetimes, wind-scaled count, zoom scatter

- MAX_AGE 8s → 25s: particles live much longer, drastically less twinkle
- activeN scales 60→300 with wind speed (0–25 kt): calm = sparse drift,
  strong wind = dense streaks
- Zoom-out detection: scatter() fires immediately when latRange grows >1.8×,
  instantly filling the new viewport instead of waiting for old particles to die
- Alpha uses sine curve (sin(π·age/MAX_AGE)) instead of linear fade:
  particles ease in from birth, peak at mid-life, and ease out — no bright
  birth flash that caused the twinkle effect

https://claude.ai/code/session_01HXPjBsogsJVRwCiekDGkJX

2 files changed, 18 insertions, 4 deletions

diff --git a/android-app/app/src/main/kotlin/org/terst/nav/MainActivity.kt b/android-app/app/src/main/kotlin/org/terst/nav/MainActivity.kt
index 996892e..e6355bd 100644
--- a/android-app/app/src/main/kotlin/org/terst/nav/MainActivity.kt
+++ b/android-app/app/src/main/kotlin/org/terst/nav/MainActivity.kt
@@ -296,6 +296,8 @@ class MainActivity : AppCompatActivity(), SafetyFragment.SafetyListener {
     private fun hideOverlays() {
         fragmentContainer.visibility = View.GONE
         fabRecordTrack.visibility = View.VISIBLE
+        // Re-apply cached conditions in case they arrived while the overlay was covering the sheet
+        lastConditions?.let { applyConditions(it) }
     }
 
     override fun onQuitRequested() {
diff --git a/android-app/app/src/main/kotlin/org/terst/nav/ui/map/ParticleWindView.kt b/android-app/app/src/main/kotlin/org/terst/nav/ui/map/ParticleWindView.kt
index 24b25db..26d9ea2 100644
--- a/android-app/app/src/main/kotlin/org/terst/nav/ui/map/ParticleWindView.kt
+++ b/android-app/app/src/main/kotlin/org/terst/nav/ui/map/ParticleWindView.kt
@@ -8,6 +8,7 @@ import android.util.AttributeSet
 import android.view.View
 import org.maplibre.android.geometry.LatLng
 import org.maplibre.android.maps.MapLibreMap
+import kotlin.math.PI
 import kotlin.math.cos
 import kotlin.math.sin
 import kotlin.random.Random
@@ -38,10 +39,12 @@ class ParticleWindView @JvmOverloads constructor(
     private var windSpeedKt = 0.0
 
     private val N = 300
+    private var activeN = 150               // wind-speed-dependent; updated in setWind()
     private val particleLat = FloatArray(N)
     private val particleLon = FloatArray(N)
     private val particleAge = FloatArray(N)
-    private val MAX_AGE = 8f   // seconds before forced respawn
+    private val MAX_AGE = 25f  // seconds before forced respawn
+    private var lastLatRange = 0f          // for zoom-out scatter detection
 
     private val paint = Paint(Paint.ANTI_ALIAS_FLAG).apply {
         strokeWidth = 3.5f
@@ -62,6 +65,8 @@ class ParticleWindView @JvmOverloads constructor(
     fun setWind(dirFromDeg: Double, speedKt: Double) {
         windDirFromDeg = dirFromDeg
         windSpeedKt = speedKt
+        // Scale particle count: 60 at calm → 300 at 25+ kt
+        activeN = (60 + (speedKt.coerceIn(0.0, 25.0) / 25.0 * 240)).toInt()
     }
 
     // ── Rendering ────────────────────────────────────────────────────────────
@@ -86,6 +91,11 @@ class ParticleWindView @JvmOverloads constructor(
         val latNorth = maxOf(nL.latitude, nR.latitude, fL.latitude, fR.latitude).toFloat()
         val latRange = latNorth - latSouth
 
+        // If the user zoomed out significantly, immediately redistribute particles
+        // across the new viewport rather than waiting for them to drift to the edges.
+        if (lastLatRange > 0f && latRange > lastLatRange * 1.8f) scatter(m)
+        lastLatRange = latRange
+
         // West edge = left screen side; east edge = right screen side.
         // Using screen-ordered corners handles antimeridian crossing correctly.
         val lonWest = minOf(nL.longitude, fL.longitude).toFloat()
@@ -109,7 +119,7 @@ class ParticleWindView @JvmOverloads constructor(
         val tailDx = sin(screenRad).toFloat() * TAIL_PX
         val tailDy = (-cos(screenRad)).toFloat() * TAIL_PX
 
-        for (i in 0 until N) {
+        for (i in 0 until activeN) {
             particleLat[i] += dlat
             particleLon[i] += dlon
             // Wrap longitude into [-180, 180] after movement
@@ -137,7 +147,9 @@ class ParticleWindView @JvmOverloads constructor(
                 LatLng(particleLat[i].toDouble(), particleLon[i].toDouble())
             )
 
-            val alpha = ((1f - particleAge[i] / MAX_AGE) * 220).toInt().coerceIn(40, 220)
+            // Sine curve: smooth fade-in from birth, peak at mid-life, smooth fade-out.
+            // No bright birth flash — particles ease in and ease out.
+            val alpha = (sin(PI * particleAge[i] / MAX_AGE) * 200).toInt().coerceIn(15, 200)
             paint.color = Color.argb(alpha, 30, 100, 255)
 
             canvas.drawLine(pt.x - tailDx, pt.y - tailDy, pt.x, pt.y, paint)
@@ -170,7 +182,7 @@ class ParticleWindView @JvmOverloads constructor(
         val lonEast = maxOf(nR.longitude, fR.longitude).toFloat()
         val lonSpan = if (lonEast >= lonWest) lonEast - lonWest else lonEast - lonWest + 360f
 
-        for (i in 0 until N) {
+        for (i in 0 until activeN) {
             particleLat[i] = latSouth + Random.nextFloat() * latRange
             var newLon = lonWest + Random.nextFloat() * lonSpan
             if (newLon > 180f) newLon -= 360f