fix(mobile): keep the Android composer placeholder on one line

apps/mobile/modules/t3-composer-editor/android/src/main/java/expo/modules/t3composereditor/T3ComposerEditorView.kt

@@ -12,6 +12,7 @@ import android.text.Editable
 import android.text.InputType
 import android.text.InputFilter
 import android.text.Spanned
+import android.text.TextUtils
 import android.text.TextWatcher
 import android.text.style.ReplacementSpan
 import android.util.TypedValue
@@ -254,7 +255,7 @@ class T3ComposerEditorView(context: Context, appContext: AppContext) : ExpoView(
   }
 
   fun setPlaceholder(placeholder: String) {
-    editor.hint = placeholder
+    editor.placeholder = placeholder
   }
 
   fun setClipboardFragment(fragment: String) {
@@ -267,12 +268,14 @@ class T3ComposerEditorView(context: Context, appContext: AppContext) : ExpoView(
     } else {
       Typeface.DEFAULT
     }
+    editor.applyPlaceholder()
   }
 
   fun setFontSize(fontSize: Float) {
     editor.textSize = fontSize
     applyLineHeight()
     applyTokenSpans()
+    editor.applyPlaceholder()
   }
 
   fun setLineHeight(lineHeight: Float) {
@@ -587,6 +590,37 @@ internal class SelectionAwareEditText(context: Context) : EditText(context) {
   var maxInputChars = Int.MAX_VALUE
   var clipboardFragment = ""
 
+  /**
+   * Placeholder shown while the draft is empty. An editable TextView never ellipsizes its hint,
+   * so a long placeholder wraps once a wide system font or a large font scale (Samsung defaults)
+   * runs out of width, and the resting composer grows to two lines. The hint is instead cut to
+   * one line with an ellipsis for whatever width the editor is measured at.
+   */
+  var placeholder = ""
+    set(value) {
+      field = value
+      applyPlaceholder()
+    }
+
+  fun applyPlaceholder(availableWidth: Int = width - compoundPaddingLeft - compoundPaddingRight) {
+    val next =
+      if (availableWidth > 0) {
+        TextUtils.ellipsize(placeholder, paint, availableWidth.toFloat(), TextUtils.TruncateAt.END)
+      } else {
+        placeholder
+      }
+    if (hint?.toString() != next.toString()) hint = next
+  }
+
+  override fun onMeasure(widthMeasureSpec: Int, heightMeasureSpec: Int) {
+    if (MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.UNSPECIFIED) {
+      applyPlaceholder(
+        MeasureSpec.getSize(widthMeasureSpec) - compoundPaddingLeft - compoundPaddingRight
+      )
+    }
+    super.onMeasure(widthMeasureSpec, heightMeasureSpec)
+  }
+
   private fun deleteChip(backwards: Boolean): Boolean {
     val content = text
     val start = minOf(selectionStart, selectionEnd)

apps/mobile/modules/t3-composer-editor/android/src/test/java/expo/modules/t3composereditor/ComposerPlaceholderTest.kt

@@ -0,0 +1,59 @@
+package expo.modules.t3composereditor
+
+import android.view.View
+import android.view.ViewGroup
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertTrue
+import org.junit.Test
+import org.junit.runner.RunWith
+import org.robolectric.RobolectricTestRunner
+import org.robolectric.RuntimeEnvironment
+import org.robolectric.annotation.Config
+import org.robolectric.annotation.GraphicsMode
+
+@RunWith(RobolectricTestRunner::class)
+@Config(sdk = [36], manifest = Config.NONE)
+@GraphicsMode(GraphicsMode.Mode.NATIVE)
+class ComposerPlaceholderTest {
+  private val placeholder = "Ask the repo agent, or run a command…"
+  private val editor = SelectionAwareEditText(RuntimeEnvironment.getApplication()).apply {
+    layoutParams = ViewGroup.LayoutParams(
+      ViewGroup.LayoutParams.MATCH_PARENT,
+      ViewGroup.LayoutParams.MATCH_PARENT,
+    )
+    textSize = 16f
+    placeholder = this@ComposerPlaceholderTest.placeholder
+  }
+
+  private fun measureAt(width: Int) {
+    editor.measure(
+      View.MeasureSpec.makeMeasureSpec(width, View.MeasureSpec.EXACTLY),
+      View.MeasureSpec.makeMeasureSpec(0, View.MeasureSpec.UNSPECIFIED),
+    )
+    editor.layout(0, 0, width, editor.measuredHeight)
+  }
+
+  @Test
+  fun narrowEditorCutsThePlaceholderToOneLine() {
+    val fullWidth = editor.paint.measureText(placeholder)
+    val width = (fullWidth / 2).toInt()
+    measureAt(width)
+
+    val hint = editor.hint.toString()
+    assertTrue(hint, hint.endsWith("…") && hint.length < placeholder.length)
+    assertTrue(hint, editor.paint.measureText(hint) <= width)
+  }
+
+  @Test
+  fun wideEditorKeepsTheWholePlaceholder() {
+    measureAt(editor.paint.measureText(placeholder).toInt() + 20)
+    assertEquals(placeholder, editor.hint.toString())
+  }
+
+  @Test
+  fun placeholderRecoversWhenTheEditorWidens() {
+    measureAt((editor.paint.measureText(placeholder) / 2).toInt())
+    measureAt(editor.paint.measureText(placeholder).toInt() + 20)
+    assertEquals(placeholder, editor.hint.toString())
+  }
+}