Extract naturalCardBaseDp and scaledDensity helpers so tests bind to real code

resolvedCardHeight previously routed through a two-branch if(isWide)
hidden inside the composable extension, so the regression tests for
the global-slider reach bug only exercised the inner resolveCardHeight
(which was never broken). Lift the aspect -> natural base mapping
into a pure naturalCardBaseDp(AspectRatio) function with an exhaustive
when, and test that directly. SQUARE and FOUR_THREE rows piggyback on
the TALL global by design; the override branch still handles their
absolute heightDp values (Music presets at 100dp, etc.).

MainContent and DisplaySizePage's preview now both call a single
scaledDensity(base, percent) helper instead of inlining Density(...)
constructions. The double-fontScale-multiply regression has a real
guard now: tests assert scaledDensity(2.0, 1.0, 150) returns density
3.0 and fontScale 1.0 (not 1.5), and a third test confirms an
accessibility fontScale survives a UI scale change.

Pure refactor + test sharpening; no behaviour change.

app/src/main/java/com/github/damontecres/wholphin/MainContent.kt

@@ -19,7 +19,6 @@ import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.graphics.RectangleShape
 import androidx.compose.ui.platform.LocalDensity
-import androidx.compose.ui.unit.Density
 import androidx.compose.ui.unit.dp
 import androidx.lifecycle.Lifecycle
 import androidx.lifecycle.compose.LifecycleEventEffect
@@ -41,6 +40,7 @@ import com.github.damontecres.wholphin.ui.setup.SwitchServerContent
 import com.github.damontecres.wholphin.ui.setup.SwitchUserContent
 import com.github.damontecres.wholphin.ui.util.InterfaceCustomization
 import com.github.damontecres.wholphin.ui.util.LocalInterfaceCustomization
+import com.github.damontecres.wholphin.ui.util.scaledDensity
 
 @Composable
 fun MainContent(
@@ -63,17 +63,13 @@ fun MainContent(
         val interfaceCustomization =
             remember(appPreferences) { InterfaceCustomization(appPreferences) }
         val baseDensity = LocalDensity.current
-        val scaledDensity =
+        val uiScaleDensity =
             remember(baseDensity, interfaceCustomization.uiScalePercent) {
-                val factor = interfaceCustomization.uiScalePercent / 100f
-                Density(
-                    density = baseDensity.density * factor,
-                    fontScale = baseDensity.fontScale,
-                )
+                scaledDensity(baseDensity, interfaceCustomization.uiScalePercent)
             }
         CompositionLocalProvider(
             LocalInterfaceCustomization provides interfaceCustomization,
-            LocalDensity provides scaledDensity,
+            LocalDensity provides uiScaleDensity,
         ) {
             NavDisplay(
                 backStack = backStack,

app/src/main/java/com/github/damontecres/wholphin/ui/preferences/displaysize/DisplaySizePage.kt

@@ -69,6 +69,7 @@ import com.github.damontecres.wholphin.ui.preferences.PreferencesViewModel
 import com.github.damontecres.wholphin.ui.preferences.SliderPreference
 import com.github.damontecres.wholphin.ui.tryRequestFocus
 import com.github.damontecres.wholphin.ui.util.ResStringProvider
+import com.github.damontecres.wholphin.ui.util.scaledDensity
 import com.github.damontecres.wholphin.ui.util.withinBoundsOrDefault
 import com.github.damontecres.wholphin.util.HomeRowLoadingState
 import dagger.hilt.android.lifecycle.HiltViewModel
@@ -184,13 +185,7 @@ fun DisplaySizePage(
             )
         }
     val previewDensity =
-        remember(systemDensity, uiScale) {
-            val factor = uiScale / 100f
-            Density(
-                density = systemDensity.density * factor,
-                fontScale = systemDensity.fontScale,
-            )
-        }
+        remember(systemDensity, uiScale) { scaledDensity(systemDensity, uiScale) }
 
     CompositionLocalProvider(LocalDensity provides pageDensity) {
         Row(

app/src/main/java/com/github/damontecres/wholphin/ui/util/InterfaceHelpers.kt

@@ -1,6 +1,7 @@
 package com.github.damontecres.wholphin.ui.util
 
 import androidx.compose.runtime.Composable
+import androidx.compose.ui.unit.Density
 import androidx.compose.ui.unit.Dp
 import androidx.compose.ui.unit.dp
 import com.github.damontecres.wholphin.data.model.HomeRowViewOptions
@@ -11,9 +12,9 @@ import com.github.damontecres.wholphin.ui.Cards
 @Composable
 internal fun HomeRowViewOptions.resolvedCardHeight(): Dp {
     val custom = LocalInterfaceCustomization.current
-    val isWide = aspectRatio == AspectRatio.WIDE
-    val naturalBase = if (isWide) Cards.HEIGHT_EPISODE else Cards.HEIGHT_2X3_DP
-    val globalForThisRow = if (isWide) custom.episodeCardHeightDp else custom.cardHeightDp
+    val naturalBase = naturalCardBaseDp(aspectRatio)
+    val globalForThisRow =
+        if (naturalBase == Cards.HEIGHT_EPISODE) custom.episodeCardHeightDp else custom.cardHeightDp
     return resolveCardHeight(
         heightDp = heightDp,
         cardSizeMultiplier = cardSizeMultiplier,
@@ -22,6 +23,14 @@ internal fun HomeRowViewOptions.resolvedCardHeight(): Dp {
     ).dp
 }
 
+// SQUARE / FOUR_THREE rows piggyback on the poster (TALL) global; they either store an
+// explicit override (Music preset rows at 100dp, etc.) or compose well enough at poster size.
+internal fun naturalCardBaseDp(aspectRatio: AspectRatio): Int =
+    when (aspectRatio) {
+        AspectRatio.WIDE -> Cards.HEIGHT_EPISODE
+        AspectRatio.TALL, AspectRatio.SQUARE, AspectRatio.FOUR_THREE -> Cards.HEIGHT_2X3_DP
+    }
+
 internal fun resolveCardHeight(
     heightDp: Int,
     cardSizeMultiplier: Int,
@@ -33,6 +42,20 @@ internal fun resolveCardHeight(
     return base * cardSizeMultiplier / 100
 }
 
+// Scales density (and therefore both dp layouts and sp text) linearly by uiScalePercent.
+// fontScale is left at the user's system value so Android's accessibility setting still
+// applies on top - scaling both would render text at factor^2 instead of factor.
+internal fun scaledDensity(
+    base: Density,
+    uiScalePercent: Int,
+): Density {
+    val factor = uiScalePercent / 100f
+    return Density(
+        density = base.density * factor,
+        fontScale = base.fontScale,
+    )
+}
+
 internal fun Int.withinBoundsOrDefault(preference: AppSliderPreference<*>): Int {
     val min = preference.min.toInt()
     val max = preference.max.toInt()

app/src/test/java/com/github/damontecres/wholphin/ui/util/InterfaceHelpersTest.kt

@@ -1,10 +1,12 @@
 package com.github.damontecres.wholphin.ui.util
 
+import androidx.compose.ui.unit.Density
 import com.github.damontecres.wholphin.data.model.HomeRowConfig
 import com.github.damontecres.wholphin.data.model.HomeRowViewOptions
 import com.github.damontecres.wholphin.preferences.AppPreference
 import com.github.damontecres.wholphin.preferences.AppPreferences
 import com.github.damontecres.wholphin.services.migrateLegacyRowSpacing
+import com.github.damontecres.wholphin.ui.AspectRatio
 import com.github.damontecres.wholphin.ui.Cards
 import org.jellyfin.sdk.model.UUID
 import org.junit.Assert.assertEquals
@@ -96,23 +98,47 @@ class InterfaceHelpersTest {
     }
 
     @Test
-    fun `UI scale must not double-apply on text`() {
-        // sp rendering = sp.value * density * fontScale. MainContent's scaled Density
-        // multiplies density by factor and leaves fontScale alone; at slider 150% text must
-        // grow by 1.5x, not 1.5*1.5 = 2.25x. Regression guard for the original bug where both
-        // were multiplied.
-        val baseDensity = 2.0f
-        val baseFontScale = 1.0f
-        val factor = 1.5f
-        // The pattern MainContent.kt uses after the fix:
-        val scaledDensity = baseDensity * factor
-        val scaledFontScale = baseFontScale
-        val baseTextPx = 14f * baseDensity * baseFontScale
-        val scaledTextPx = 14f * scaledDensity * scaledFontScale
-        assertEquals(1.5f * baseTextPx, scaledTextPx, 0.001f)
-        // Double-application (the bug) would have produced 2.25x.
-        val buggyTextPx = 14f * (baseDensity * factor) * (baseFontScale * factor)
-        assertEquals(2.25f * baseTextPx, buggyTextPx, 0.001f)
+    fun `naturalCardBaseDp maps WIDE to episode base`() {
+        assertEquals(Cards.HEIGHT_EPISODE, naturalCardBaseDp(AspectRatio.WIDE))
+    }
+
+    @Test
+    fun `naturalCardBaseDp maps TALL, SQUARE, FOUR_THREE to poster base`() {
+        // SQUARE and FOUR_THREE piggyback on the poster global. Music preset rows (SQUARE)
+        // store their own absolute heightDp = 100 so they still hit the override branch -
+        // the natural base just supplies the fallback if a row ever stores no override.
+        assertEquals(Cards.HEIGHT_2X3_DP, naturalCardBaseDp(AspectRatio.TALL))
+        assertEquals(Cards.HEIGHT_2X3_DP, naturalCardBaseDp(AspectRatio.SQUARE))
+        assertEquals(Cards.HEIGHT_2X3_DP, naturalCardBaseDp(AspectRatio.FOUR_THREE))
+    }
+
+    @Test
+    fun `scaledDensity scales density linearly and leaves fontScale alone`() {
+        val base = Density(density = 2.0f, fontScale = 1.0f)
+        val scaled = scaledDensity(base, 150)
+        // density grows by 1.5 - so dp layouts scale 1.5x.
+        assertEquals(3.0f, scaled.density, 0.001f)
+        // fontScale stays put - sp text is (density * fontScale) so it also scales linearly.
+        // Scaling fontScale too would render text at 1.5^2 = 2.25x.
+        assertEquals(1.0f, scaled.fontScale, 0.001f)
+    }
+
+    @Test
+    fun `scaledDensity at 100 percent returns identity`() {
+        val base = Density(density = 2.0f, fontScale = 1.2f)
+        val scaled = scaledDensity(base, 100)
+        assertEquals(base.density, scaled.density, 0.001f)
+        assertEquals(base.fontScale, scaled.fontScale, 0.001f)
+    }
+
+    @Test
+    fun `scaledDensity preserves user accessibility fontScale across UI scale changes`() {
+        // A user with system fontScale 1.3x (accessibility setting) bumping UI scale to 75%
+        // should keep fontScale = 1.3 - the UI scale slider is independent of accessibility.
+        val base = Density(density = 2.0f, fontScale = 1.3f)
+        val scaled = scaledDensity(base, 75)
+        assertEquals(1.5f, scaled.density, 0.001f)
+        assertEquals(1.3f, scaled.fontScale, 0.001f)
     }
 
     @Test