Keep pre-mounting within half of a 30 Hz frame

A display the system slows below 60 Hz can spend more of each frame
pre-mounting, so the ceiling is now half of a 30 Hz frame (16.67 ms)
rather than half of a 60 Hz one: from 30 Hz up the budget is half of the
display's frame, and below 30 Hz it is 16.67 ms. A display that reports
no refresh rate is still budgeted as a 60 Hz one.

Author: YevheniiKotyrlo

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/fabric/mounting/MountItemDispatcher.kt

@@ -341,10 +341,12 @@ internal class MountItemDispatcher(
     dispatchPreMountItemsImpl(deadline)
   }
 
-  /** Half of the display's current frame, and never longer than [LONGEST_PRE_MOUNT_NANOS]. */
+  /**
+   * Half of the display's current frame, and never longer than [LONGEST_PRE_MOUNT_NANOS]. A display
+   * that reports no refresh rate is budgeted as a 60 Hz one.
+   */
   private fun computePreMountBudgetNanos(): Long {
-    val frameIntervalNanos =
-        frameIntervalSource.frameIntervalNanos() ?: return LONGEST_PRE_MOUNT_NANOS
+    val frameIntervalNanos = frameIntervalSource.frameIntervalNanos() ?: FRAME_TIME_NS
     return minOf(frameIntervalNanos / 2, LONGEST_PRE_MOUNT_NANOS)
   }
 
@@ -435,11 +437,13 @@ internal class MountItemDispatcher(
   private companion object {
     private const val TAG: String = "MountItemDispatcher"
 
+    private const val FRAME_TIME_NS: Long = (1000000000 / 60).toLong()
+
     /**
-     * The longest a frame pre-mounts for: half of a 60 Hz frame, so that below 60 Hz pre-mounting
-     * holds the UI thread no longer than it does at 60 Hz.
+     * The longest a frame pre-mounts for: half of a 30 Hz frame, so that a display below 30 Hz,
+     * such as an adaptive panel idling at 1 Hz, holds the UI thread no longer than at 30 Hz.
      */
-    private const val LONGEST_PRE_MOUNT_NANOS: Long = 1_000_000_000L / 60 / 2
+    private const val LONGEST_PRE_MOUNT_NANOS: Long = 1_000_000_000L / 30 / 2
 
     private fun <E> drainConcurrentItemQueue(queue: Queue<E>): List<E>? {
       if (queue.isEmpty()) {

packages/react-native/ReactAndroid/src/test/java/com/facebook/react/fabric/mounting/MountItemDispatcherTest.kt

@@ -89,18 +89,30 @@ class MountItemDispatcherTest {
       }
 
   @Test
-  fun preMountsForHalfOfTheDisplaysFrameFromSixtyHertzUp() {
+  fun preMountsForHalfOfTheDisplaysFrameFromThirtyHertzUp() {
     // Items start at 0, 1, 2, ... ms, and the first one starting past half of the frame waits for
-    // the next frame: half of a 60 Hz frame is 8.33 ms, of a 185 Hz frame 2.70 ms.
-    assertThat(preMountFrameAtEachRefreshRate(listOf(60, 90, 120, 144, 165, 185, 240)))
-        .isEqualTo(mapOf(60 to 9, 90 to 6, 120 to 5, 144 to 4, 165 to 4, 185 to 3, 240 to 3))
+    // the next frame: half of a 30 Hz frame is 16.67 ms, of a 185 Hz frame 2.70 ms.
+    assertThat(preMountFrameAtEachRefreshRate(listOf(30, 48, 60, 90, 120, 144, 165, 185, 240)))
+        .isEqualTo(
+            mapOf(
+                30 to 17,
+                48 to 11,
+                60 to 9,
+                90 to 6,
+                120 to 5,
+                144 to 4,
+                165 to 4,
+                185 to 3,
+                240 to 3,
+            )
+        )
   }
 
   @Test
-  fun neverPreMountsLongerThanHalfOfASixtyHertzFrameBelowSixtyHertz() {
-    // Half of a 1 Hz frame would be 500 ms; every rate below 60 Hz keeps 8.33 ms.
-    assertThat(preMountFrameAtEachRefreshRate(listOf(1, 10, 24, 30, 48)))
-        .isEqualTo(mapOf(1 to 9, 10 to 9, 24 to 9, 30 to 9, 48 to 9))
+  fun neverPreMountsLongerThanHalfOfAThirtyHertzFrameBelowThirtyHertz() {
+    // Half of a 1 Hz frame would be 500 ms; every rate below 30 Hz keeps 16.67 ms.
+    assertThat(preMountFrameAtEachRefreshRate(listOf(1, 10, 24)))
+        .isEqualTo(mapOf(1 to 17, 10 to 17, 24 to 17))
   }
 
   @Test