fix: preserve CarPlay physical size across browser viewport changes

common/src/main/java/com/shilapi/xcertplay/CarPlayHostActivity.kt

@@ -2689,8 +2689,10 @@ class CarPlayHostActivity : ComponentActivity() {
         AirPlayDisplaySettings.resolvePhysicalSizeMm(
             currentWidthPixels = size.width,
             currentHeightPixels = size.height,
-            maximumWidthPixels = maxOf(maximumDetectedWidthPixels, size.width),
-            maximumHeightPixels = maxOf(maximumDetectedHeightPixels, size.height),
+            // The browser canvas fills its display at every pixel resolution. Treating it as
+            // a fraction of a previous viewport shrinks the reported screen and enlarges iOS UI.
+            maximumWidthPixels = size.width,
+            maximumHeightPixels = size.height,
             referenceMillimeters = widthPhysicalMm,
             basis = physicalSizeBasis,
         )

common/src/test/java/com/shilapi/xcertplay/CarPlayBrowserDisplaySizeTest.kt

@@ -0,0 +1,56 @@
+package com.shilapi.xcertplay
+
+import com.shilapi.xcertplay.airplay.AirPlayPhysicalSizeBasis
+import com.shilapi.xcertplay.airplay.AirPlayPhysicalSizeMm
+import com.shilapi.xcertplay.airplay.CarPlaySize
+import org.junit.Assert.assertEquals
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.robolectric.Robolectric
+import org.robolectric.RobolectricTestRunner
+import org.robolectric.annotation.Config
+import org.robolectric.util.ReflectionHelpers
+
+@RunWith(RobolectricTestRunner::class)
+@Config(sdk = [29])
+class CarPlayBrowserDisplaySizeTest {
+    private fun host(): CarPlayHostActivity =
+        Robolectric.buildActivity(CarPlayHostActivity::class.java).get().also {
+            ReflectionHelpers.setField(it, "maximumDetectedWidthPixels", 3840)
+            ReflectionHelpers.setField(it, "maximumDetectedHeightPixels", 2160)
+            ReflectionHelpers.setField(it, "widthPhysicalMm", CarPlaySize.MEDIUM.widthMillimeters)
+        }
+
+    private fun physicalSize(host: CarPlayHostActivity, width: Int, height: Int): AirPlayPhysicalSizeMm {
+        val sizeClass = CarPlayHostActivity::class.java.declaredClasses.single { it.simpleName == "DisplaySize" }
+        val size = sizeClass.getDeclaredConstructor(Int::class.javaPrimitiveType, Int::class.javaPrimitiveType)
+            .apply { isAccessible = true }.newInstance(width, height)
+        return CarPlayHostActivity::class.java.getDeclaredMethod("resolvePhysicalSize", sizeClass)
+            .apply { isAccessible = true }.invoke(host, size) as AirPlayPhysicalSizeMm
+    }
+
+    @Test fun mediumBrowserCanvasKeepsItsPhysicalSizeAfterAFourKViewport() {
+        assertEquals(AirPlayPhysicalSizeMm(300, 233), physicalSize(host(), 1184, 920))
+    }
+
+    @Test fun resizingTheBrowserOrReducingPixelsDoesNotEnlargeCarPlayControls() {
+        val host = host()
+        assertEquals(AirPlayPhysicalSizeMm(300, 169), physicalSize(host, 1920, 1080))
+        assertEquals(AirPlayPhysicalSizeMm(300, 233), physicalSize(host, 1184, 920))
+        assertEquals(AirPlayPhysicalSizeMm(300, 233), physicalSize(host, 592, 460))
+    }
+
+    @Test fun eachCarPlaySizePresetKeepsItsConfiguredWidth() {
+        val host = host()
+        for (preset in CarPlaySize.entries) {
+            ReflectionHelpers.setField(host, "widthPhysicalMm", preset.widthMillimeters)
+            assertEquals(preset.widthMillimeters, physicalSize(host, 1184, 920).widthMm)
+        }
+    }
+
+    @Test fun aSavedHeightReferenceAlsoKeepsItsConfiguredLength() {
+        val host = host()
+        ReflectionHelpers.setField(host, "physicalSizeBasis", AirPlayPhysicalSizeBasis.HEIGHT)
+        assertEquals(AirPlayPhysicalSizeMm(386, 300), physicalSize(host, 1184, 920))
+    }
+}