From 1c61573c483f1b9429720e87f5b40e8645af3aa8 Mon Sep 17 00:00:00 2001
From: Sebastian Markbage <sebastian@calyptus.eu>
Date: Fri, 6 Jun 2025 11:24:42 -0400
Subject: [PATCH] Shorten throttle to hit a specific target metric

---
 ...tDOMFizzInstructionSetInlineCodeStrings.js |  4 +--
 .../ReactDOMFizzInstructionSetShared.js       | 34 +++++++++++++++----
 2 files changed, 30 insertions(+), 8 deletions(-)

diff --git a/packages/react-dom-bindings/src/server/fizz-instruction-set/ReactDOMFizzInstructionSetInlineCodeStrings.js b/packages/react-dom-bindings/src/server/fizz-instruction-set/ReactDOMFizzInstructionSetInlineCodeStrings.js
index 72a5aba4fb3..6cfb4b61eda 100644
--- a/packages/react-dom-bindings/src/server/fizz-instruction-set/ReactDOMFizzInstructionSetInlineCodeStrings.js
+++ b/packages/react-dom-bindings/src/server/fizz-instruction-set/ReactDOMFizzInstructionSetInlineCodeStrings.js
@@ -6,9 +6,9 @@ export const markShellTime =
 export const clientRenderBoundary =
   '$RX=function(b,c,d,e,f){var a=document.getElementById(b);a&&(b=a.previousSibling,b.data="$!",a=a.dataset,c&&(a.dgst=c),d&&(a.msg=d),e&&(a.stck=e),f&&(a.cstck=f),b._reactRetry&&b._reactRetry())};';
 export const completeBoundary =
-  '$RB=[];$RV=function(c){$RT=performance.now();for(var a=0;a<c.length;a+=2){var b=c[a],h=c[a+1],e=b.parentNode;if(e){var f=b.previousSibling,g=0;do{if(b&&8===b.nodeType){var d=b.data;if("/$"===d||"/&"===d)if(0===g)break;else g--;else"$"!==d&&"$?"!==d&&"$~"!==d&&"$!"!==d&&"&"!==d||g++}d=b.nextSibling;e.removeChild(b);b=d}while(b);for(;h.firstChild;)e.insertBefore(h.firstChild,b);f.data="$";f._reactRetry&&f._reactRetry()}}c.length=0};$RC=function(c,a){if(a=document.getElementById(a))if(a.parentNode.removeChild(a),c=document.getElementById(c))c.previousSibling.data="$~",$RB.push(c,a),2===$RB.length&&setTimeout($RV.bind(null,$RB),("number"!==typeof $RT?0:$RT)+300-performance.now())};';
+  '$RB=[];$RV=function(b){$RT=performance.now();for(var a=0;a<b.length;a+=2){var c=b[a],h=b[a+1],e=c.parentNode;if(e){var f=c.previousSibling,g=0;do{if(c&&8===c.nodeType){var d=c.data;if("/$"===d||"/&"===d)if(0===g)break;else g--;else"$"!==d&&"$?"!==d&&"$~"!==d&&"$!"!==d&&"&"!==d||g++}d=c.nextSibling;e.removeChild(c);c=d}while(c);for(;h.firstChild;)e.insertBefore(h.firstChild,c);f.data="$";f._reactRetry&&f._reactRetry()}}b.length=0};$RC=function(b,a){if(a=document.getElementById(a))if(a.parentNode.removeChild(a),b=document.getElementById(b))b.previousSibling.data="$~",$RB.push(b,a),2===$RB.length&&(b="number"!==typeof $RT?0:$RT,a=performance.now(),setTimeout($RV.bind(null,$RB),2300>a&&2E3<a?2300-a:b+300-a))};';
 export const completeBoundaryUpgradeToViewTransitions =
-  '$RV=function(z,g){function k(a,b){var e=a.getAttribute(b);e&&(b=a.style,l.push(a,b.viewTransitionName,b.viewTransitionClass),"auto"!==e&&(b.viewTransitionClass=e),(a=a.getAttribute("vt-name"))||(a="_T_"+K++ +"_"),b.viewTransitionName=a,A=!0)}var A=!1,K=0,l=[];try{var f=document.__reactViewTransition;if(f){f.finished.finally($RV.bind(null,g));return}var m=new Map;for(f=1;f<g.length;f+=2)for(var h=g[f].querySelectorAll("[vt-share]"),d=0;d<h.length;d++){var c=h[d];m.set(c.getAttribute("vt-name"),c)}var t=[];for(h=0;h<g.length;h+=2){var B=g[h],w=B.parentNode;if(w){var u=w.getBoundingClientRect();if(u.left||u.top||u.width||u.height){c=B;for(f=0;c;){if(8===c.nodeType){var q=c.data;if("/$"===q)if(0===f)break;else f--;else"$"!==q&&"$?"!==q&&"$~"!==q&&"$!"!==q||f++}else if(1===c.nodeType){d=c;var C=d.getAttribute("vt-name"),x=m.get(C);k(d,x?"vt-share":"vt-exit");x&&(k(x,"vt-share"),m.set(C,null));var D=d.querySelectorAll("[vt-share]");for(d=0;d<D.length;d++){var E=D[d],F=E.getAttribute("vt-name"),\nG=m.get(F);G&&(k(E,"vt-share"),k(G,"vt-share"),m.set(F,null))}}c=c.nextSibling}for(var H=g[h+1],r=H.firstElementChild;r;)null!==m.get(r.getAttribute("vt-name"))&&k(r,"vt-enter"),r=r.nextElementSibling;c=w;do for(var n=c.firstElementChild;n;){var I=n.getAttribute("vt-update");I&&"none"!==I&&!l.includes(n)&&k(n,"vt-update");n=n.nextElementSibling}while((c=c.parentNode)&&1===c.nodeType&&"none"!==c.getAttribute("vt-update"));t.push.apply(t,H.querySelectorAll(\'img[src]:not([loading="lazy"])\'))}}}if(A){var y=\ndocument.__reactViewTransition=document.startViewTransition({update:function(){z(g);for(var a=[document.documentElement.clientHeight,document.fonts.ready],b={},e=0;e<t.length;b={g:b.g},e++)if(b.g=t[e],!b.g.complete){var p=b.g.getBoundingClientRect();0<p.bottom&&0<p.right&&p.top<window.innerHeight&&p.left<window.innerWidth&&(p=new Promise(function(v){return function(J){v.g.addEventListener("load",J);v.g.addEventListener("error",J)}}(b)),a.push(p))}return Promise.race([Promise.all(a),new Promise(function(v){return setTimeout(v,\n500)})])},types:[]});y.ready.finally(function(){for(var a=l.length-3;0<=a;a-=3){var b=l[a],e=b.style;e.viewTransitionName=l[a+1];e.viewTransitionClass=l[a+1];""===b.getAttribute("style")&&b.removeAttribute("style")}});y.finished.finally(function(){document.__reactViewTransition===y&&(document.__reactViewTransition=null)});$RB=[];return}}catch(a){}z(g)}.bind(null,$RV);';
+  '$RV=function(A,g){function k(a,b){var e=a.getAttribute(b);e&&(b=a.style,l.push(a,b.viewTransitionName,b.viewTransitionClass),"auto"!==e&&(b.viewTransitionClass=e),(a=a.getAttribute("vt-name"))||(a="_T_"+K++ +"_"),b.viewTransitionName=a,B=!0)}var B=!1,K=0,l=[];try{var f=document.__reactViewTransition;if(f){f.finished.finally($RV.bind(null,g));return}var m=new Map;for(f=1;f<g.length;f+=2)for(var h=g[f].querySelectorAll("[vt-share]"),d=0;d<h.length;d++){var c=h[d];m.set(c.getAttribute("vt-name"),c)}var u=[];for(h=0;h<g.length;h+=2){var C=g[h],x=C.parentNode;if(x){var v=x.getBoundingClientRect();if(v.left||v.top||v.width||v.height){c=C;for(f=0;c;){if(8===c.nodeType){var r=c.data;if("/$"===r)if(0===f)break;else f--;else"$"!==r&&"$?"!==r&&"$~"!==r&&"$!"!==r||f++}else if(1===c.nodeType){d=c;var D=d.getAttribute("vt-name"),y=m.get(D);k(d,y?"vt-share":"vt-exit");y&&(k(y,"vt-share"),m.set(D,null));var E=d.querySelectorAll("[vt-share]");for(d=0;d<E.length;d++){var F=E[d],G=F.getAttribute("vt-name"),\nH=m.get(G);H&&(k(F,"vt-share"),k(H,"vt-share"),m.set(G,null))}}c=c.nextSibling}for(var I=g[h+1],t=I.firstElementChild;t;)null!==m.get(t.getAttribute("vt-name"))&&k(t,"vt-enter"),t=t.nextElementSibling;c=x;do for(var n=c.firstElementChild;n;){var J=n.getAttribute("vt-update");J&&"none"!==J&&!l.includes(n)&&k(n,"vt-update");n=n.nextElementSibling}while((c=c.parentNode)&&1===c.nodeType&&"none"!==c.getAttribute("vt-update"));u.push.apply(u,I.querySelectorAll(\'img[src]:not([loading="lazy"])\'))}}}if(B){var z=\ndocument.__reactViewTransition=document.startViewTransition({update:function(){A(g);for(var a=[document.documentElement.clientHeight,document.fonts.ready],b={},e=0;e<u.length;b={g:b.g},e++)if(b.g=u[e],!b.g.complete){var p=b.g.getBoundingClientRect();0<p.bottom&&0<p.right&&p.top<window.innerHeight&&p.left<window.innerWidth&&(p=new Promise(function(w){return function(q){w.g.addEventListener("load",q);w.g.addEventListener("error",q)}}(b)),a.push(p))}return Promise.race([Promise.all(a),new Promise(function(w){var q=\nperformance.now();setTimeout(w,2300>q&&2E3<q?2300-q:500)})])},types:[]});z.ready.finally(function(){for(var a=l.length-3;0<=a;a-=3){var b=l[a],e=b.style;e.viewTransitionName=l[a+1];e.viewTransitionClass=l[a+1];""===b.getAttribute("style")&&b.removeAttribute("style")}});z.finished.finally(function(){document.__reactViewTransition===z&&(document.__reactViewTransition=null)});$RB=[];return}}catch(a){}A(g)}.bind(null,$RV);';
 export const completeBoundaryWithStyles =
   '$RM=new Map;$RR=function(n,w,p){function u(q){this._p=null;q()}for(var r=new Map,t=document,h,b,e=t.querySelectorAll("link[data-precedence],style[data-precedence]"),v=[],k=0;b=e[k++];)"not all"===b.getAttribute("media")?v.push(b):("LINK"===b.tagName&&$RM.set(b.getAttribute("href"),b),r.set(b.dataset.precedence,h=b));e=0;b=[];var l,a;for(k=!0;;){if(k){var f=p[e++];if(!f){k=!1;e=0;continue}var c=!1,m=0;var d=f[m++];if(a=$RM.get(d)){var g=a._p;c=!0}else{a=t.createElement("link");a.href=d;a.rel=\n"stylesheet";for(a.dataset.precedence=l=f[m++];g=f[m++];)a.setAttribute(g,f[m++]);g=a._p=new Promise(function(q,x){a.onload=u.bind(a,q);a.onerror=u.bind(a,x)});$RM.set(d,a)}d=a.getAttribute("media");!g||d&&!matchMedia(d).matches||b.push(g);if(c)continue}else{a=v[e++];if(!a)break;l=a.getAttribute("data-precedence");a.removeAttribute("media")}c=r.get(l)||h;c===h&&(h=a);r.set(l,a);c?c.parentNode.insertBefore(a,c.nextSibling):(c=t.head,c.insertBefore(a,c.firstChild))}if(p=document.getElementById(n))p.previousSibling.data=\n"$~";Promise.all(b).then($RC.bind(null,n,w),$RX.bind(null,n,"CSS failed to load"))};';
 export const completeSegment =
diff --git a/packages/react-dom-bindings/src/server/fizz-instruction-set/ReactDOMFizzInstructionSetShared.js b/packages/react-dom-bindings/src/server/fizz-instruction-set/ReactDOMFizzInstructionSetShared.js
index 8c1f70bb8ae..74e054077cf 100644
--- a/packages/react-dom-bindings/src/server/fizz-instruction-set/ReactDOMFizzInstructionSetShared.js
+++ b/packages/react-dom-bindings/src/server/fizz-instruction-set/ReactDOMFizzInstructionSetShared.js
@@ -13,8 +13,16 @@ const SUSPENSE_PENDING_START_DATA = '$?';
 const SUSPENSE_QUEUED_START_DATA = '$~';
 const SUSPENSE_FALLBACK_START_DATA = '$!';
 
+const FALLBACK_THROTTLE_MS = 300;
+
 const SUSPENSEY_FONT_AND_IMAGE_TIMEOUT = 500;
 
+// If you have a target goal in mind for a metric to hit, you don't want the
+// only reason you miss it by a little bit to be throttling heuristics.
+// This tries to avoid throttling if avoiding it would let you hit this metric.
+// This is derived from trying to hit an LCP of 2.5 seconds with some head room.
+const TARGET_VANITY_METRIC = 2300;
+
 // TODO: Symbols that are referenced outside this module use dynamic accessor
 // notation instead of dot notation to prevent Closure's advanced compilation
 // mode from renaming. We could use extern files instead, but I couldn't get it
@@ -290,9 +298,18 @@ export function revealCompletedBoundariesWithViewTransitions(
           }
           return Promise.race([
             Promise.all(blockingPromises),
-            new Promise(resolve =>
-              setTimeout(resolve, SUSPENSEY_FONT_AND_IMAGE_TIMEOUT),
-            ),
+            new Promise(resolve => {
+              const currentTime = performance.now();
+              const msUntilTimeout =
+                // If the throttle would make us miss the target metric, then shorten the throttle.
+                // performance.now()'s zero value is assumed to be the start time of the metric.
+                currentTime < TARGET_VANITY_METRIC &&
+                currentTime > TARGET_VANITY_METRIC - FALLBACK_THROTTLE_MS
+                  ? TARGET_VANITY_METRIC - currentTime
+                  : // Otherwise it's throttled starting from last commit time.
+                    SUSPENSEY_FONT_AND_IMAGE_TIMEOUT;
+              setTimeout(resolve, msUntilTimeout);
+            }),
           ]);
         },
         types: [], // TODO: Add a hard coded type for Suspense reveals.
@@ -360,8 +377,6 @@ export function clientRenderBoundary(
   }
 }
 
-const FALLBACK_THROTTLE_MS = 300;
-
 export function completeBoundary(suspenseBoundaryID, contentID) {
   const contentNodeOuter = document.getElementById(contentID);
   if (!contentNodeOuter) {
@@ -395,8 +410,15 @@ export function completeBoundary(suspenseBoundaryID, contentID) {
     // to flush the batch. This is delayed by the throttle heuristic.
     const globalMostRecentFallbackTime =
       typeof window['$RT'] !== 'number' ? 0 : window['$RT'];
+    const currentTime = performance.now();
     const msUntilTimeout =
-      globalMostRecentFallbackTime + FALLBACK_THROTTLE_MS - performance.now();
+      // If the throttle would make us miss the target metric, then shorten the throttle.
+      // performance.now()'s zero value is assumed to be the start time of the metric.
+      currentTime < TARGET_VANITY_METRIC &&
+      currentTime > TARGET_VANITY_METRIC - FALLBACK_THROTTLE_MS
+        ? TARGET_VANITY_METRIC - currentTime
+        : // Otherwise it's throttled starting from last commit time.
+          globalMostRecentFallbackTime + FALLBACK_THROTTLE_MS - currentTime;
     // We always schedule the flush in a timer even if it's very low or negative to allow
     // for multiple completeBoundary calls that are already queued to have a chance to
     // make the batch.