Bug 1603621 - Expose partial present usage to about:support r=nical

Differential Revision: https://phabricator.services.mozilla.com/D57074

--HG--
extra : moz-landing-system : lando

gfx/config/gfxFeature.h

@@ -27,6 +27,7 @@ namespace gfx {
   _(WEBRENDER, Feature, "WebRender")                              \
   _(WEBRENDER_QUALIFIED, Feature, "WebRender qualified")          \
   _(WEBRENDER_COMPOSITOR, Feature, "WebRender native compositor") \
+  _(WEBRENDER_PARTIAL, Feature, "WebRender partial present")      \
   _(OMTP, Feature, "Off Main Thread Painting")                    \
   _(ADVANCED_LAYERS, Feature, "Advanced Layers")                  \
   _(WEBGPU, Feature, "WebGPU")                                    \

gfx/thebes/gfxPlatform.cpp

@@ -3331,6 +3331,20 @@ void gfxPlatform::InitWebRenderConfig() {
     featureComp.UserEnable("Enabled");
   }
 
+  // Initialize WebRender partial present config.
+  // It is used only for reporting to Decision Log.
+  if (StaticPrefs::gfx_webrender_max_partial_present_rects_AtStartup() > 0) {
+    // Partial present is used only when WebRender compositor is not used.
+    if (UseWebRender() && !gfxVars::UseWebRenderCompositor()) {
+      FeatureState& featurePartial =
+          gfxConfig::GetFeature(Feature::WEBRENDER_PARTIAL);
+      featurePartial.EnableByDefault();
+      // Call UserEnable() only for reporting to Decision Log.
+      // If feature is enabled by default. It is not reported to Decision Log.
+      featurePartial.UserEnable("Enabled");
+    }
+  }
+
   // Set features that affect WR's RendererOptions
   gfxVars::SetUseGLSwizzle(
       IsFeatureSupported(nsIGfxInfo::FEATURE_GL_SWIZZLE, true));