fix(view): keep the horizontal scroll when a prop repeats

Nitro resends an object prop whenever its JavaScript identity changes, and an
inline literal is a new object on every render. The example Editor screen
re-renders on each scroll event for its status line, so contentInsets reached
the native setter per frame, relayout zeroed the widest line and the clamp
snapped scrollX to 0: a horizontal swipe flickered between the finger and
column 0. The hybrid now ignores theme and contentInsets values equal to the
current ones, relayout measures the visible lines again and restores the
horizontal scroll, the example hoists its insets literal, and an instrumented
test scrolls right, changes the insets and expects scrollX unchanged.

Author: SectionTN

android/src/main/java/com/margelo/nitro/codepad/HybridCodeEditorView.kt

@@ -56,6 +56,7 @@ class HybridCodeEditorView(val context: ThemedReactContext) : HybridCodeEditorVi
 
   override var theme: NativeTheme? = null
     set(value) {
+      if (value == field) return
       field = value
       editor.setTheme(value?.let { toColors(it) } ?: ThemeColors.default)
     }
@@ -116,6 +117,7 @@ class HybridCodeEditorView(val context: ThemedReactContext) : HybridCodeEditorVi
 
   override var contentInsets: NativeInsets? = null
     set(value) {
+      if (value == field) return
       field = value
       editor.setContentInsets(
         (value?.top ?: 0.0).toFloat(),

android/src/main/java/me/sectiontn/codepad/view/CodeEditorView.kt

@@ -452,15 +452,27 @@ class CodeEditorView(context: Context) : View(context) {
   private fun relayout() {
     anchorDirty = true
     val anchor = viewport.anchor()
+    val scrollX = viewport.scrollX
     viewport.rowHeight = rowMetrics().rowHeight
     cache.clear()
     heights.resetAll()
     viewport.widestLine = 0
     updateGutter()
     viewport.restore(anchor)
+    // The visible lines are measured again right away so the horizontal position survives the widest line reset.
+    layoutVisibleLines()
+    viewport.scrollTo(scrollX, viewport.scrollY)
     invalidate()
   }
 
+  private fun layoutVisibleLines() {
+    var line = viewport.firstVisibleLine()
+    while (line < core.document.lineCount && line <= viewport.lastVisibleLine()) {
+      if (cache[line] == null) layoutLine(line)
+      line++
+    }
+  }
+
   private fun updateGutter(): Boolean {
     val w = if (lineNumbers) {
       GutterMetrics.width(core.document.lineCount, paint.measureText("0"), dp(GUTTER_PADDING_DP).toFloat())
@@ -501,6 +513,7 @@ class CodeEditorView(context: Context) : View(context) {
     val entry = Entry(layout, null, -1)
     cache.put(line, entry)
     viewport.measured(line, layout.rowCount)
+    if (!viewport.wrap) viewport.widestLine = maxOf(viewport.widestLine, Math.round(layout.width()))
     return entry
   }
 

example/android/app/src/androidTest/java/codepad/example/ScrollTest.kt

@@ -0,0 +1,33 @@
+package codepad.example
+
+import android.os.SystemClock
+import android.view.KeyEvent
+import androidx.test.ext.junit.runners.AndroidJUnit4
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertTrue
+import org.junit.Test
+import org.junit.runner.RunWith
+
+@RunWith(AndroidJUnit4::class)
+class ScrollTest {
+  @Test
+  fun insetChangesKeepTheHorizontalScroll() {
+    launch().use { s ->
+      val editor = s.editor()
+      onMain {
+        editor.setLineNumbers(false)
+        editor.setInitialText("x".repeat(200))
+        layout(editor)
+        draw(editor)
+        val now = SystemClock.uptimeMillis()
+        editor.dispatchKeyEvent(KeyEvent(now, now, KeyEvent.ACTION_DOWN, KeyEvent.KEYCODE_MOVE_END, 0))
+        draw(editor)
+        val before = editor.frame.scrollX
+        assertTrue("the caret reveal scrolled right, scrollX $before", before > 0f)
+        editor.setContentInsets(0f, 24f, 0f, 0f)
+        draw(editor)
+        assertEquals("scrollX after the inset change", before, editor.frame.scrollX, 0.5f)
+      }
+    }
+  }
+}

example/src/screens/EditorScreen.tsx

@@ -24,6 +24,8 @@ const block = [
 
 const sample = new Array(40).fill(block).join('\n');
 
+// Object props keep one identity across renders; a fresh literal would reach native on every scroll event.
+const INSETS = { bottom: 24 };
 const SUGGESTIONS: Suggestions[] = ['off', 'forceOff', 'on'];
 
 function Toggle({ label, value, onPress }: { label: string; value: boolean; onPress: () => void }) {
@@ -90,7 +92,7 @@ export function EditorScreen() {
         highlightActiveLine={activeLine}
         placeholder="Start typing"
         scrollEnabled
-        contentInsets={{ bottom: 24 }}
+        contentInsets={INSETS}
         suggestions={suggestions}
         autoCloseBrackets={autoClose}
         onFocus={() => setFocused(true)}