ui: scale the auto frame cap margin with the display refresh rate

include/rex/ui/window.h

@@ -222,6 +222,15 @@ class Window {
   // readable from any thread (frame pacers, settings UI).
   static float CachedDisplayRefreshHz();
 
+  // Refresh-derived automatic frame cap for the given display refresh rate,
+  // 0 while the refresh is unknown. Presents run with tearing allowed, so on
+  // a VRR display any present arriving faster than the panel's minimum
+  // refresh period tears; the cap must sit far enough below the refresh rate
+  // that swap-to-present jitter cannot cross it. A fixed margin that is
+  // ample at 60 Hz is too thin at 144 Hz, so the margin scales with the
+  // refresh rate. Single source of the policy for pacers and settings UI.
+  static float AutoFrameCapHz(float refresh_hz);
+
   // Round trips are not guaranteed to return the same results.
   static constexpr uint32_t ConvertSizeDpi(uint32_t size, uint32_t new_dpi, uint32_t old_dpi) {
     // Always rounding up to prevent zero sizes (unless the input is zero) as

src/ui/overlay/simple_settings_overlay.cpp

@@ -1351,10 +1351,12 @@ void SimpleSettingsDialog::BuildRows(std::vector<RowSpec>& rows, int category) {
             "Auto tracks the current display and does exactly that. Frames above the "
             "refresh rate cannot be shown and only make steady motion judder.";
         if (FrameCapHasAuto()) {
-          const float hz = rex::ui::Window::CachedDisplayRefreshHz();
+          // Same policy the guest pacer applies (see Window::AutoFrameCapHz).
+          const float auto_cap =
+              rex::ui::Window::AutoFrameCapHz(rex::ui::Window::CachedDisplayRefreshHz());
           row.options.push_back(
-              hz >= 30.0f
-                  ? "Auto (" + std::to_string(int(hz) - 4) + " FPS)"
+              auto_cap > 0.0f
+                  ? "Auto (" + std::to_string(int(auto_cap)) + " FPS)"
                   : std::string("Auto"));
         }
         for (size_t i = 1; i < kFrameCapLabels.size(); ++i) {

src/ui/window.cpp

@@ -11,6 +11,7 @@
 
 #include <algorithm>
 #include <atomic>
+#include <cmath>
 #include <iterator>
 
 #include <rex/assert.h>
@@ -78,6 +79,17 @@ void Window::UpdateCachedDisplayRefresh() {
   }
 }
 
+float Window::AutoFrameCapHz(float refresh_hz) {
+  if (refresh_hz < 30.0f) {
+    return 0.0f;
+  }
+  // 4 FPS of margin or 5% of the refresh rate, whichever is larger (see the
+  // declaration): 4 FPS is 6.7% of headroom at 60 Hz but only 0.2 ms of
+  // present slack at 144 Hz, within routine thread-scheduling jitter.
+  const float margin = std::max(4.0f, refresh_hz * 0.05f);
+  return std::floor(refresh_hz - margin);
+}
+
 Window::Window(WindowedAppContext& app_context, const std::string_view title,
                uint32_t desired_logical_width, uint32_t desired_logical_height)
     : app_context_(app_context),