Ask the headset for a configured refresh rate, 120 Hz on the Steam Frame

[vr] refresh_rate (Hz, 0 = the headset's own) is requested through
XR_FB_display_refresh_rate each time the session starts running and
whenever the setting changes, matched against the runtime's own list
within half a hertz. Setting it back to 0 restores the rate the session
started at. A rate that is not offered, or one the runtime declines, is
logged and changes nothing.

The game renders 60 frames a second, so at 120 Hz every frame shows for
exactly two refreshes, where 72 or 90 Hz hold some frames longer than
others. The Steam Frame build defaults to 120; every other build keeps
the headset's own rate unless the player picks one.

The F10/headset panel and the Android launcher's Headset section offer
the choice; the interpolation tooltip points at it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019HBRGKTE1GnN2ah8gcZKr3

runtime/tests/frame_interpolation_pacing_tests.cpp

@@ -12,6 +12,13 @@ static void Require(bool condition) {
 }
 
 int main() {
+    // [vr] refresh_rate picks the runtime's own value for the rate asked.
+    const float frame_rates[] = {72.0f, 90.0f, 119.98f, 144.0f};
+    Require(mkw::vr::MatchDisplayRefreshRate(frame_rates, 4, 120.0f) == 119.98f);
+    Require(mkw::vr::MatchDisplayRefreshRate(frame_rates, 4, 90.0f) == 90.0f);
+    Require(mkw::vr::MatchDisplayRefreshRate(frame_rates, 4, 60.0f) == 0.0f);
+    Require(mkw::vr::MatchDisplayRefreshRate(frame_rates, 4, 100.0f) == 0.0f);
+    Require(mkw::vr::MatchDisplayRefreshRate(frame_rates, 0, 120.0f) == 0.0f);
     for (uint32_t target : {0u, 1u, 72u, 90u, 120u}) {
         std::istringstream input("[vr]\nframe_interpolation_fps = " + std::to_string(target) + "\n");
         const auto config = RuntimeConfigFile::ParseConfig(input);