More documentation about warm-up frames (flutter#132085)

packages/flutter/lib/src/scheduler/binding.dart

@@ -933,6 +933,30 @@ mixin SchedulerBinding on BindingBase {
   /// [scheduleWarmUpFrame] was already called, this call will be ignored.
   ///
   /// Prefer [scheduleFrame] to update the display in normal operation.
+  ///
+  /// ## Design discussion
+  ///
+  /// The Flutter engine prompts the framework to generate frames when it
+  /// receives a request from the operating system (known for historical reasons
+  /// as a vsync). However, this may not happen for several milliseconds after
+  /// the app starts (or after a hot reload). To make use of the time between
+  /// when the widget tree is first configured and when the engine requests an
+  /// update, the framework schedules a _warm-up frame_.
+  ///
+  /// A warm-up frame may never actually render (as the engine did not request
+  /// it and therefore does not have a valid context in which to paint), but it
+  /// will cause the framework to go through the steps of building, laying out,
+  /// and painting, which can together take several milliseconds. Thus, when the
+  /// engine requests a real frame, much of the work will already have been
+  /// completed, and the framework can generate the frame with minimal
+  /// additional effort.
+  ///
+  /// Warm-up frames are scheduled by [runApp] on startup, and by
+  /// [RendererBinding.performReassemble] during a hot reload.
+  ///
+  /// Warm-up frames are also scheduled when the framework is unblocked by a
+  /// call to [RendererBinding.allowFirstFrame] (corresponding to a call to
+  /// [RendererBinding.deferFirstFrame] that blocked the rendering).
   void scheduleWarmUpFrame() {
     if (_warmUpFrame || schedulerPhase != SchedulerPhase.idle) {
       return;