js+css: implement HTMLInputElement.indeterminate and :indeterminate

FIDELITY.md

@@ -18,6 +18,16 @@ The committed PNGs under `testdata/renders/` back every claim here. Reproduce
 them with the commands at the bottom. The measured-vs-Chrome numbers live in
 [`bench/REPORT.md`](bench/REPORT.md).
 
+## 2026-09-28 (round 138) — `HTMLInputElement.indeterminate` and the `:indeterminate` CSS pseudo-class were entirely missing — CONFIRMED real corpus usage on github.com's own "select all" bulk-action checkboxes (engine#238)
+
+Broadened the corpus sweep to a genuinely fresh page (github.com's own homepage and its `behaviors.js`/`element-registry.js`/`environment.js` bundles, not previously in this session's ten-page set), after the existing corpus stopped turning up new evidenced gaps. `.indeterminate=` was the one new real hit: `(0,v.l)("[data-indeterminate]",{constructor:HTMLInputElement,initialize(e){e.indeterminate=!0}})` — GitHub's own tri-state "select all" checkbox pattern.
+
+- **Read the bibliography first, not assumed symmetric with `checked`**: HTML Standard §4.10.5.1.19 — `indeterminate` has NO backing content attribute at all, unlike `checked`. It is pure script-set runtime state with no HTML markup equivalent.
+- **Fixed across three layers, matching that architectural difference**: a new `dom.Node.Indeterminate bool` field (the same shape as the existing `Quirks`/`Shadow` runtime-only fields, not `Attr`) backs both a new `js/dom.go` `.indeterminate` accessor (reads/writes the field directly, NOT via `setAttr`/`removeAttr`) and a new `:indeterminate` CSS pseudo-class in `css/selector.go` (`compound.Indeterminate`, wired into `Matches`/the parser switch/the specificity count/the bare-compound-is-non-empty check — the same four sites `:disabled` already uses).
+- **Deliberately left out of scope, disclosed honestly**: this engine's own checkbox painter (`paint.paintCheckboxLike`) already approximates `checked` as a plain accent-coloured fill with no real checkmark glyph — adding a visually distinct indeterminate mark (a real browser's usual dash) would mean inventing new iconography beyond what `checked` itself has, not extending an existing one. This round only makes `.indeterminate`/`:indeterminate` correct for script logic and AUTHOR CSS (`input:indeterminate { ... }` rules), not for this engine's own default checkbox appearance.
+- **Two new tests**, git-stash-confirmed (a `go vet` compile failure, the strongest class): `css/selector_test.go`'s `TestIndeterminatePseudo` (mirroring `TestCheckedPseudo`/`TestDisabledEnabledPseudo`'s exact structure) and `js/js_test.go`'s `TestIndeterminateAccessor` (confirms setting it does NOT touch `getAttribute('indeterminate')`, unlike every other boolean accessor on this element).
+- **Bench**: exactly flat/within already-documented noise across all ten pages, including github.com/golang/go itself (0.638, unchanged) — expected, since this round's fix is a pure JS/CSS-correctness change with no paint-visible effect by design (see the scoping note above). All coverage floors held.
+
 ## 2026-09-27 (round 137) — `scrollTop`/`scrollLeft` were hardcoded to always read 0 with no setter at all, so a script-set value silently did nothing — CONFIRMED real corpus usage on pkg.go.dev's own scroll-into-view logic, which both sets AND reads the value back to decide what to do next (engine#237)
 
 Continued sweeping this session's own two hottest bundles for `.propertyName =` assignments across the FULL corpus this time (not just the two hottest files), broadened past boolean properties to numeric/string ones too. `.scrollTop=` stood out on pkg.go.dev's own `frontend.js`, in three separate call sites of a "keep the active item visible in a scrollable list" routine: `f<e.scrollTop?e.scrollTop=f:p>e.scrollTop+e.clientHeight&&(e.scrollTop=p-e.c…)`.

bench/REPORT.md

@@ -2,7 +2,7 @@
 
 <!-- AUTO-GENERATED: this header, the results table and the montage list are regenerated by `go run ./cmd/compare`. The hand-written analysis below the BEGIN ANALYSIS marker is preserved across re-runs. -->
 
-**Generated:** 2026-09-27 19:44 UTC  
+**Generated:** 2026-09-28 06:24 UTC  
 **Viewport:** 1024×768, device-scale 1  
 **Timing:** median of N=5 runs after 1 warmup, wall-clock incl. network fetch  
 **Chrome:** `/Applications/Google Chrome.app/Contents/MacOS/Google Chrome`  
@@ -11,16 +11,16 @@
 <!-- BEGIN RESULTS TABLE -->
 | URL | SSIM | pixdiff % | webengine ms | chrome ms | speed× | region | status |
 |-----|-----:|----------:|-------------:|----------:|-------:|:------:|:------:|
-| example.com/ | 0.954 | 1.5 | 51.2 | 1283.7 | 25.09 | 1024×768 | ok |
-| en.wikipedia.org/wiki/Go_(programming_language) | 0.436 | 21.3 | 2711.9 | 3739.4 | 1.38 | 1024×2500 | ok |
-| pkg.go.dev/net/http | 0.716 | 11.3 | 5170.6 | 3565.4 | 0.69 | 1024×2500 | ok |
-| go.dev/blog/ | 0.690 | 16.7 | 4259.3 | 1917.9 | 0.45 | 1024×1439 | ok |
-| react.dev/ | 0.725 | 33.0 | 3248.1 | 2174.9 | 0.67 | 1024×2500 | ok |
-| news.ycombinator.com/ | 0.602 | 14.4 | 1258.2 | 2159.4 | 1.72 | 1024×1077 | ok |
-| developer.mozilla.org/en-US/docs/Web/CSS | 0.606 | 18.1 | 771.5 | 1469.1 | 1.90 | 1009×2500 | ok |
-| github.com/golang/go | 0.638 | 14.2 | 2603.9 | 2124.3 | 0.82 | 1024×2491 | ok |
-| tailwindcss.com/ | 0.725 | 12.2 | 7412.4 | 1920.9 | 0.26 | 1024×2500 | ok |
-| caniuse.com/ | 0.657 | 18.3 | 4691.5 | 3343.0 | 0.71 | 1024×1740 | ok |
+| example.com/ | 0.954 | 1.5 | 52.3 | 1283.0 | 24.52 | 1024×768 | ok |
+| en.wikipedia.org/wiki/Go_(programming_language) | 0.436 | 21.3 | 2699.3 | 3880.0 | 1.44 | 1024×2500 | ok |
+| pkg.go.dev/net/http | 0.716 | 11.3 | 5461.5 | 3951.6 | 0.72 | 1024×2500 | ok |
+| go.dev/blog/ | 0.690 | 16.7 | 4432.9 | 1995.8 | 0.45 | 1024×1439 | ok |
+| react.dev/ | 0.725 | 33.0 | 3436.6 | 2266.9 | 0.66 | 1024×2500 | ok |
+| news.ycombinator.com/ | 0.603 | 14.4 | 1233.1 | 2098.7 | 1.70 | 1024×1077 | ok |
+| developer.mozilla.org/en-US/docs/Web/CSS | 0.606 | 18.1 | 540.9 | 1522.4 | 2.81 | 1009×2500 | ok |
+| github.com/golang/go | 0.638 | 14.2 | 2607.3 | 1733.7 | 0.66 | 1024×2491 | ok |
+| tailwindcss.com/ | 0.740 | 11.3 | 7475.3 | 1868.8 | 0.25 | 1024×2500 | ok |
+| caniuse.com/ | 0.657 | 18.2 | 4899.5 | 3228.2 | 0.66 | 1024×1740 | ok |
 <!-- END RESULTS TABLE -->
 
 Speed× is `chrome_ms / webengine_ms`: >1 means webengine is faster.

bench/results.json

@@ -1,9 +1,9 @@
 [
   {
     "url": "https://example.com/",
-    "webengine_ms": 51.159,
-    "chrome_ms": 1283.69,
-    "speed_ratio": 25.092163646670187,
+    "webengine_ms": 52.324,
+    "chrome_ms": 1282.957,
+    "speed_ratio": 24.519474810794286,
     "ssim": 0.9541451179596087,
     "pixdiff_pct": 1.53350830078125,
     "region_w": 1024,
@@ -14,9 +14,9 @@
   },
   {
     "url": "https://en.wikipedia.org/wiki/Go_(programming_language)",
-    "webengine_ms": 2711.893,
-    "chrome_ms": 3739.355,
-    "speed_ratio": 1.3788726177618364,
+    "webengine_ms": 2699.322,
+    "chrome_ms": 3879.953,
+    "speed_ratio": 1.4373805718621193,
     "ssim": 0.4361758822140753,
     "pixdiff_pct": 21.279570312500002,
     "region_w": 1024,
@@ -27,9 +27,9 @@
   },
   {
     "url": "https://pkg.go.dev/net/http",
-    "webengine_ms": 5170.566,
-    "chrome_ms": 3565.42,
-    "speed_ratio": 0.6895608720592679,
+    "webengine_ms": 5461.531,
+    "chrome_ms": 3951.595,
+    "speed_ratio": 0.7235324673612582,
     "ssim": 0.7155686319990039,
     "pixdiff_pct": 11.3370703125,
     "region_w": 1024,
@@ -40,9 +40,9 @@
   },
   {
     "url": "https://go.dev/blog/",
-    "webengine_ms": 4259.27,
-    "chrome_ms": 1917.945,
-    "speed_ratio": 0.4502989948981867,
+    "webengine_ms": 4432.901,
+    "chrome_ms": 1995.772,
+    "speed_ratio": 0.4502180400599968,
     "ssim": 0.6901862268272003,
     "pixdiff_pct": 16.726839384989574,
     "region_w": 1024,
@@ -53,9 +53,9 @@
   },
   {
     "url": "https://react.dev/",
-    "webengine_ms": 3248.095,
-    "chrome_ms": 2174.916,
-    "speed_ratio": 0.6695974101742714,
+    "webengine_ms": 3436.649,
+    "chrome_ms": 2266.886,
+    "speed_ratio": 0.6596210436387306,
     "ssim": 0.72456508425769,
     "pixdiff_pct": 32.97015625,
     "region_w": 1024,
@@ -66,11 +66,11 @@
   },
   {
     "url": "https://news.ycombinator.com/",
-    "webengine_ms": 1258.212,
-    "chrome_ms": 2159.361,
-    "speed_ratio": 1.7162139607633689,
-    "ssim": 0.602218626514641,
-    "pixdiff_pct": 14.379044074976788,
+    "webengine_ms": 1233.055,
+    "chrome_ms": 2098.676,
+    "speed_ratio": 1.7020132921889128,
+    "ssim": 0.6033293622898777,
+    "pixdiff_pct": 14.418306058495823,
     "region_w": 1024,
     "region_h": 1077,
     "montage": "out/news.ycombinator.com.png",
@@ -79,9 +79,9 @@
   },
   {
     "url": "https://developer.mozilla.org/en-US/docs/Web/CSS",
-    "webengine_ms": 771.542,
-    "chrome_ms": 1469.097,
-    "speed_ratio": 1.9041050260387637,
+    "webengine_ms": 540.939,
+    "chrome_ms": 1522.391,
+    "speed_ratio": 2.8143487528168616,
     "ssim": 0.6056768401997266,
     "pixdiff_pct": 18.121902874132807,
     "region_w": 1009,
@@ -92,11 +92,11 @@
   },
   {
     "url": "https://github.com/golang/go",
-    "webengine_ms": 2603.943,
-    "chrome_ms": 2124.291,
-    "speed_ratio": 0.815797811242412,
-    "ssim": 0.6384297325703495,
-    "pixdiff_pct": 14.22907623695303,
+    "webengine_ms": 2607.312,
+    "chrome_ms": 1733.675,
+    "speed_ratio": 0.6649280945280043,
+    "ssim": 0.63813074167239,
+    "pixdiff_pct": 14.235309614612603,
     "region_w": 1024,
     "region_h": 2491,
     "montage": "out/github.com_golang_go.png",
@@ -105,11 +105,11 @@
   },
   {
     "url": "https://tailwindcss.com/",
-    "webengine_ms": 7412.4,
-    "chrome_ms": 1920.871,
-    "speed_ratio": 0.25914292267012035,
-    "ssim": 0.7254186601435486,
-    "pixdiff_pct": 12.207265625,
+    "webengine_ms": 7475.3,
+    "chrome_ms": 1868.777,
+    "speed_ratio": 0.24999357885302262,
+    "ssim": 0.7396458938927728,
+    "pixdiff_pct": 11.30765625,
     "region_w": 1024,
     "region_h": 2500,
     "montage": "out/tailwindcss.com.png",
@@ -118,11 +118,11 @@
   },
   {
     "url": "https://caniuse.com/",
-    "webengine_ms": 4691.492,
-    "chrome_ms": 3342.994,
-    "speed_ratio": 0.7125652137955261,
-    "ssim": 0.657124719073104,
-    "pixdiff_pct": 18.255208333333332,
+    "webengine_ms": 4899.46,
+    "chrome_ms": 3228.213,
+    "speed_ratio": 0.6588915921346434,
+    "ssim": 0.6569742927777596,
+    "pixdiff_pct": 18.19190014367816,
     "region_w": 1024,
     "region_h": 1740,
     "montage": "out/caniuse.com.png",

css/selector.go

@@ -54,6 +54,14 @@ type compound struct {
 	// real-world need — only the element's own attribute is.
 	Disabled bool
 	Enabled  bool
+	// Indeterminate is set by the ":indeterminate" pseudo-class. Unlike
+	// Checked/Disabled above, there is no content attribute to read at
+	// static render time — "indeterminate" is a pure script-set runtime
+	// state (see dom.Node.Indeterminate's own doc comment) — so this simply
+	// mirrors that field directly, with no "at static render time" caveat:
+	// a script that sets it before this cascade runs is exactly the real
+	// case (see round 138's own corpus trigger).
+	Indeterminate bool
 	// FirstChild is set by the ":first-child" structural pseudo-class — the
 	// element has no preceding element sibling. Unlike ":nth-child(...)" this
 	// is cheap and common enough (and, critically, common as a ":not()"
@@ -372,6 +380,9 @@ func (c compound) matches(n *dom.Node) bool {
 	if c.Enabled && isDisabled(n) {
 		return false
 	}
+	if c.Indeterminate && !n.Indeterminate {
+		return false
+	}
 	if c.FirstChild && prevElementSibling(n) != nil {
 		return false
 	}
@@ -519,6 +530,9 @@ func (c compound) specificity() (idCount, classCount, tagCount int) {
 	if c.Enabled {
 		classCount++ // ":enabled" is a pseudo-class (class-level weight)
 	}
+	if c.Indeterminate {
+		classCount++ // ":indeterminate" is a pseudo-class (class-level weight)
+	}
 	if c.Host {
 		classCount++ // ":host" is itself a pseudo-class (class-level weight)
 	}
@@ -1263,6 +1277,8 @@ func parseSimple(s string) (compound, bool) {
 			c.Disabled = true
 		case "enabled":
 			c.Enabled = true
+		case "indeterminate":
+			c.Indeterminate = true
 		case "first-child":
 			c.FirstChild = true
 		case "last-child":
@@ -1394,7 +1410,7 @@ func parseSimple(s string) (compound, bool) {
 	// ":checked"/":first-child"/":not(...)"/attribute/":host" selectors carry a
 	// real constraint on their own.
 	if c.Tag == "" && c.ID == "" && len(c.Classes) == 0 &&
-		!c.Root && !c.Dynamic && !c.Checked && !c.Disabled && !c.Enabled && !c.FirstChild && !c.LastChild && !c.Empty && !c.Host && c.Part == "" && len(c.Not) == 0 && len(c.Attrs) == 0 && !c.HasPresent && !c.NthChildSet &&
+		!c.Root && !c.Dynamic && !c.Checked && !c.Disabled && !c.Enabled && !c.Indeterminate && !c.FirstChild && !c.LastChild && !c.Empty && !c.Host && c.Part == "" && len(c.Not) == 0 && len(c.Attrs) == 0 && !c.HasPresent && !c.NthChildSet &&
 		!c.OnlyChild && !c.FirstOfType && !c.LastOfType && !c.OnlyOfType && !c.NthOfTypeSet && !c.NthLastChildSet && !c.NthLastOfTypeSet {
 		return compound{}, false
 	}

css/selector_test.go

@@ -260,6 +260,44 @@ func TestDisabledEnabledPseudo(t *testing.T) {
 	}
 }
 
+// TestIndeterminatePseudo covers ":indeterminate" — entirely missing before
+// this fix, unlike ":checked"/":disabled" above it has NO backing content
+// attribute at all (dom.Node.Indeterminate is pure script-set runtime state,
+// see its own doc comment), so a real checkbox is matched by setting the
+// Node field directly rather than via an Attr map, mirroring how a script
+// would set `.indeterminate = true` at runtime. Real corpus trigger:
+// github.com's own behaviors.js sets this on its "select all" bulk-action
+// checkboxes.
+func TestIndeterminatePseudo(t *testing.T) {
+	on := &dom.Node{Type: dom.Element, Tag: "input", Attr: map[string]string{"type": "checkbox"}, Indeterminate: true}
+	off := &dom.Node{Type: dom.Element, Tag: "input", Attr: map[string]string{"type": "checkbox"}}
+
+	sel, ok := parseComplex("input:indeterminate")
+	if !ok {
+		t.Fatal("input:indeterminate should parse")
+	}
+	if !sel.Matches(on) {
+		t.Error(":indeterminate should match a checkbox with Indeterminate set")
+	}
+	if sel.Matches(off) {
+		t.Error(":indeterminate should NOT match a plain checkbox")
+	}
+
+	// A bare ":indeterminate" is a valid, real constraint on its own.
+	bare, ok := parseComplex(":indeterminate")
+	if !ok || bare.parts[0].Indeterminate != true {
+		t.Fatalf("bare :indeterminate = %+v ok=%v", bare, ok)
+	}
+	if !bare.Matches(on) || bare.Matches(off) {
+		t.Error("bare :indeterminate match wrong")
+	}
+
+	// ":indeterminate" contributes class-level specificity, same as ":checked".
+	if got := bare.Specificity(); got != 100 {
+		t.Errorf(":indeterminate specificity = %d, want 100", got)
+	}
+}
+
 func TestNotPseudo(t *testing.T) {
 	box := el("div", "", "box")
 	other := el("div", "", "other")

dom/dom.go

@@ -63,6 +63,17 @@ type Node struct {
 	// to centre (see css/ua.go's quirks-mode table rule).
 	Quirks bool
 
+	// Indeterminate is an <input type=checkbox>'s own "indeterminate" IDL
+	// state (HTML Standard §4.10.5.1.19): unlike Checked, it has NO backing
+	// content attribute at all — script is the only way to set it, so it
+	// lives here as a plain runtime field, the same shape as Quirks/Shadow
+	// above, rather than in Attr. Confirmed real usage: github.com's own
+	// behaviors.js sets `checkbox.indeterminate = true` on page load for any
+	// `[data-indeterminate]`-marked checkbox (its "select all" bulk-action
+	// tri-state pattern). Backs the JS `.indeterminate` accessor and the
+	// `:indeterminate` CSS pseudo-class (css/selector.go).
+	Indeterminate bool
+
 	// Shadow is the shadow root attached to this element (a declarative
 	// <template shadowrootmode> hoisted out at parse time — see
 	// attachDeclarativeShadowRoots), or nil for a plain element. When set,

js/dom.go

@@ -558,6 +558,17 @@ func (b *binder) defineElement(o *goja.Object, n *dom.Node) {
 				b.removeAttr(n, "checked")
 			}
 		})
+	// indeterminate: unlike checked, this has NO backing content attribute at
+	// all (HTML Standard §4.10.5.1.19) — pure script-set runtime state, held
+	// directly on dom.Node (see its own doc comment) rather than reflected
+	// through setAttr/removeAttr like every other accessor on this element.
+	// Entirely missing before this fix. Real corpus usage confirmed on
+	// github.com's own behaviors.js, which sets `checkbox.indeterminate =
+	// true` on page load for its "select all" bulk-action tri-state
+	// checkboxes (`[data-indeterminate]`).
+	b.accessor(o, "indeterminate",
+		func() goja.Value { return b.vm.ToValue(n.Indeterminate) },
+		func(v goja.Value) { n.Indeterminate = v.ToBoolean() })
 	// HTMLDetailsElement.open — a plain boolean reflection (HTML Standard
 	// §4.11.1), the same presence-based shape as checked/hidden above — was
 	// entirely missing, so `details.open = true` from script silently

js/js_test.go

@@ -671,6 +671,29 @@ func TestScrollTopLeftReadWhatYouWrote(t *testing.T) {
 		"afterSet top=42 left=7")
 }
 
+// TestIndeterminateAccessor confirms `.indeterminate` — entirely missing
+// before this fix — round-trips a script-set value. Unlike checked/disabled,
+// it has NO backing content attribute (HTML Standard §4.10.5.1.19): setting
+// it must NOT touch getAttribute('indeterminate') at all, only the runtime
+// state the accessor (and the ":indeterminate" CSS pseudo-class,
+// css/selector_test.go) reads. Real corpus usage confirmed on github.com's
+// own behaviors.js, which sets it on page load for its "select all"
+// bulk-action tri-state checkboxes.
+func TestIndeterminateAccessor(t *testing.T) {
+	_, logs, _ := runJS(t, page(`
+		var d = document.getElementById('d');
+		console.log('default='+d.indeterminate);
+		d.indeterminate = true;
+		console.log('afterSet='+d.indeterminate+' attr='+d.getAttribute('indeterminate'));
+		d.indeterminate = false;
+		console.log('afterUnset='+d.indeterminate);
+	`))
+	mustHave(t, logs,
+		"default=false",
+		"afterSet=true attr=null",
+		"afterUnset=false")
+}
+
 // TestAddEventListenerOnce confirms addEventListener's `{once: true}` option
 // — entirely ignored before this fix (the third argument was never read at
 // all) — per the DOM standard's own "inner invoke" algorithm (§2.9): a once