Android: a hardware keyboard's arrow keys move the cursor (#6199)

Scribe #6199 "Android: a hardware keyboard's arrow keys move focus instead
of the cursor".

Compose's text field sends the arrow keys of any device reporting D-pad
keys to focus search before the field sees them, and a physical keyboard
reports exactly that, so left and right jumped to the neighbouring control.
Cancelling focus search in those directions hands the key back to the field.
Up and down move between a field's lines, and from its first or last line
on to the field above or below; single-line checklist rows still step row
to row.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

android/app/src/main/java/com/fabledsword/inkwell/ui/BlockBody.kt

@@ -20,7 +20,10 @@ import androidx.compose.material3.Text
 import androidx.compose.runtime.Composable
 import androidx.compose.runtime.CompositionLocalProvider
 import androidx.compose.runtime.LaunchedEffect
+import androidx.compose.runtime.getValue
+import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.remember
+import androidx.compose.runtime.setValue
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.focus.FocusRequester
@@ -28,6 +31,7 @@ import androidx.compose.ui.focus.focusRequester
 import androidx.compose.ui.focus.onFocusChanged
 import androidx.compose.ui.graphics.SolidColor
 import androidx.compose.ui.res.stringResource
+import androidx.compose.ui.text.TextLayoutResult
 import androidx.compose.ui.text.TextStyle
 import androidx.compose.ui.text.input.ImeAction
 import androidx.compose.ui.text.input.TextFieldValue
@@ -241,6 +245,15 @@ private fun BlockField(
     keyboardActions: KeyboardActions = KeyboardActions.Default,
 ) {
     val style = textStyle.copy(color = MaterialTheme.colorScheme.onSurface)
+    // Kept for the arrow keys, which ask which line the caret is on. Read only when
+    // one is pressed, never while composing.
+    var layout by remember { mutableStateOf<TextLayoutResult?>(null) }
+    val line = { offset: Int -> layout?.getLineForOffset(offset) ?: 0 }
+    val arrows =
+        Modifier.arrowKeysMoveCaret(
+            atTop = { line(value.selection.end) == 0 },
+            atBottom = { line(value.selection.end) == (layout?.lineCount ?: 1) - 1 },
+        )
     Box(modifier = modifier) {
         if (hint != null && value.text.isEmpty()) {
             Text(
@@ -252,12 +265,13 @@ private fun BlockField(
         BasicTextField(
             value = value,
             onValueChange = onValueChange,
-            modifier = Modifier.fillMaxWidth(),
+            modifier = Modifier.fillMaxWidth().then(arrows),
             enabled = enabled,
             singleLine = singleLine,
             textStyle = style,
             keyboardOptions = keyboardOptions,
             keyboardActions = keyboardActions,
+            onTextLayout = { layout = it },
             cursorBrush = SolidColor(MaterialTheme.colorScheme.primary),
         )
     }

android/app/src/main/java/com/fabledsword/inkwell/ui/PlainField.kt

@@ -13,6 +13,8 @@ import androidx.compose.material3.TextFieldColors
 import androidx.compose.material3.TextFieldDefaults
 import androidx.compose.runtime.Composable
 import androidx.compose.ui.Modifier
+import androidx.compose.ui.focus.FocusRequester
+import androidx.compose.ui.focus.focusProperties
 import androidx.compose.ui.graphics.Color
 import androidx.compose.ui.res.stringResource
 import androidx.compose.ui.text.TextStyle
@@ -54,7 +56,7 @@ fun PlainTextField(
     TextField(
         value = value,
         onValueChange = onValueChange,
-        modifier = modifier.fillMaxWidth(),
+        modifier = modifier.fillMaxWidth().arrowKeysMoveCaret(atTop = { singleLine }, atBottom = { singleLine }),
         enabled = enabled,
         placeholder = hint?.let { { Text(stringResource(it)) } },
         singleLine = singleLine,
@@ -86,3 +88,29 @@ private fun plainFieldColors(): TextFieldColors =
         unfocusedIndicatorColor = Color.Transparent,
         disabledIndicatorColor = Color.Transparent,
     )
+
+/**
+ * Arrow keys on a hardware keyboard move the caret, not the focus.
+ *
+ * Compose's text field gives the arrow keys of any device that reports D-pad keys
+ * to focus search before the field sees them, and a physical keyboard with arrow
+ * keys reports exactly that — so without this, ← and → jump to whatever sits beside
+ * the field instead of moving through the text. Cancelling the search in a
+ * direction hands the key back: the search reports that nothing moved, and the
+ * field takes the key as editing.
+ *
+ * ← and → never leave the field. ↑ and ↓ move between its lines, and from the first
+ * or last line — [atTop], [atBottom] — on to the field above or below, the way a
+ * caret leaves one paragraph for the next. Both are asked at the moment of the key
+ * press, so they can read where the caret is now.
+ */
+fun Modifier.arrowKeysMoveCaret(
+    atTop: () -> Boolean,
+    atBottom: () -> Boolean,
+): Modifier =
+    focusProperties {
+        left = FocusRequester.Cancel
+        right = FocusRequester.Cancel
+        if (!atTop()) up = FocusRequester.Cancel
+        if (!atBottom()) down = FocusRequester.Cancel
+    }