Merge remote-tracking branch 'origin/master' into claude/issue-57

# Conflicts:
#	project.cpp

additional/dom-webgpu.js

@@ -256,12 +256,17 @@ struct UIDispatchHeader {
 @group(1) @binding(0) var outTex  : texture_storage_2d<rgba8unorm, write>;
 @group(1) @binding(1) var prevTex : texture_2d<f32>;
 
+// Renderer-reserved high flag bit (mirrors ui-shared.glsl::UI_FLAG_CLIP).
+const UI_FLAG_CLIP : u32 = 0x80000000u;
+
 fn uiResolvePixel(coord: vec2<u32>) -> bool {
     if (coord.x >= hdr.surfaceW || coord.y >= hdr.surfaceH) { return false; }
+    if ((hdr.flags & UI_FLAG_CLIP) != 0u) {
         let fx = f32(coord.x); let fy = f32(coord.y);
         if (fx < hdr.clipX || fy < hdr.clipY) { return false; }
         if (fx >= hdr.clipX + hdr.clipW) { return false; }
         if (fy >= hdr.clipY + hdr.clipH) { return false; }
+    }
     return true;
 }
 

implementations/Crafter.Graphics-Font.cpp

@@ -87,6 +87,28 @@ std::uint32_t Font::GetLineWidth(const std::string_view text, float size) {
     return lineWidth;
 }
 
+std::size_t Font::NearestCursorByte(std::string_view text, float size, float target) {
+    if (target <= 0.0f) return 0;
+    float scale = stbtt_ScaleForPixelHeight(&font, size);
+    std::size_t best     = 0;        // byte offset 0 → caret before the first glyph
+    float       bestDist = target;   // |target - 0|, and target > 0 here
+    float       cumWidth = 0.0f;
+    std::size_t i = 0;
+    while (i < text.size()) {
+        std::uint32_t cp = DecodeUtf8(text, i);   // i advances to the next boundary
+        if (cp == 0) break;
+        cumWidth += (int)(AdvanceUnits(cp) * scale);  // match GetLineWidth's per-glyph rounding
+        float d = std::abs(target - cumWidth);
+        if (d < bestDist) {
+            bestDist = d;
+            best     = i;            // byte offset just past this codepoint
+        } else {
+            break;                   // monotonic advance ⇒ already past the closest boundary
+        }
+    }
+    return best;
+}
+
 float Font::LineHeight(float size) {
     float scale = stbtt_ScaleForPixelHeight(&font, size);
     return (ascent - descent + lineGap) * scale;

implementations/Crafter.Graphics-InputField.cpp

@@ -117,23 +117,12 @@ std::size_t Crafter::InputField_HitTestCursor(const InputField& f,
                                               Font& font, float fontSize,
                                               const InputFieldColors& colors)
 {
+    // Single O(n) cumulative-advance pass over the value, keyed at codepoint
+    // byte boundaries — matching cursorPos, which is a byte offset. The old
+    // loop re-walked the prefix for every boundary (O(n^2) glyph metric
+    // lookups) and could even land mid-codepoint on multibyte input.
     float target = clickX - (rect.x + colors.paddingX);
-    if (target <= 0.0f) return 0;
+    return font.NearestCursorByte(f.value, fontSize, target);
-
-    std::size_t best     = 0;
-    float       bestDist = std::abs(target);
-    for (std::size_t i = 1; i <= f.value.size(); ++i) {
-        std::string_view sub(f.value.data(), i);
-        float w = static_cast<float>(font.GetLineWidth(sub, fontSize));
-        float d = std::abs(target - w);
-        if (d < bestDist) {
-            bestDist = d;
-            best     = i;
-        } else {
-            break;
-        }
-    }
-    return best;
 }
 
 void Crafter::DrawInputField(UIBuffer& buf, const InputField& f, Rect rect,

implementations/Crafter.Graphics-UI-Shared.cpp

@@ -38,11 +38,27 @@ UIDispatchHeader UIRenderer::FillHeader(std::uint32_t itemBufferSlot,
 #else
     h.frameIdx  = 0;
 #endif
-    h.flags     = flags;
+    h.flags     = ClipFlags(clipRectPx, window_->width, window_->height, flags);
     h._pad      = 0;
     return h;
 }
 
+std::uint32_t UIRenderer::ClipFlags(std::array<float,4> clipRectPx,
+                                    std::uint32_t surfaceWidth,
+                                    std::uint32_t surfaceHeight,
+                                    std::uint32_t flags) noexcept {
+    // The reserved kUIFlagClip bit signals "clip rect is narrower than the
+    // surface". When the rect already covers the whole surface (the common
+    // {0,0,1e9,1e9} default), the four per-pixel clip compares in
+    // uiResolveScreenPixel can never reject an in-surface pixel, so we leave
+    // the bit clear and the shader skips them — pixel-identical output.
+    const bool clipCoversSurface =
+        clipRectPx[0] <= 0.0f && clipRectPx[1] <= 0.0f
+        && clipRectPx[0] + clipRectPx[2] >= static_cast<float>(surfaceWidth)
+        && clipRectPx[1] + clipRectPx[3] >= static_cast<float>(surfaceHeight);
+    return clipCoversSurface ? (flags & ~kUIFlagClip) : (flags | kUIFlagClip);
+}
+
 std::uint32_t UIRenderer::ShapeText(Font& font, float pxSize,
                                     float x, float baselineY,
                                     std::string_view utf8,

interfaces/Crafter.Graphics-Font.cppm

@@ -60,6 +60,15 @@ namespace Crafter {
         stbtt_fontinfo font;
         Font(const std::filesystem::path& font);
         std::uint32_t GetLineWidth(const std::string_view text, float size);
+        // Map a target advance (px from the line start) to the nearest UTF-8
+        // codepoint boundary, returning its BYTE offset into `text`. A single
+        // left-to-right pass accumulates per-glyph advances (O(n) metric
+        // lookups), instead of re-walking the prefix for every boundary as a
+        // naive caller would. Cumulative advance is monotonic (advances are
+        // non-negative), so the scan stops at the first boundary past the
+        // closest one. Returned offsets land on codepoint boundaries, which are
+        // exactly the valid positions for a byte-based text cursor.
+        std::size_t NearestCursorByte(std::string_view text, float size, float target);
         float LineHeight(float size);
         float AscentPx(float size);
         float ScaleForSize(float size);

interfaces/Crafter.Graphics-UI.cppm

@@ -59,6 +59,12 @@ export namespace Crafter {
     };
     static_assert(sizeof(UIDispatchHeader) == 48);
 
+    // Reserved `flags` bit (mirrors shaders/ui-shared.glsl::UI_FLAG_CLIP).
+    // FillHeader sets it when the clip rect is narrower than the surface so
+    // the standard shaders can skip the per-pixel clip compares otherwise.
+    // The remaining low bits stay free for user-defined feature flags.
+    inline constexpr std::uint32_t kUIFlagClip = 0x80000000u;
+
     // ─── standard item PODs (match GLSL std430) ─────────────────────────
     struct QuadItem {
         float x, y, w, h;
@@ -165,6 +171,18 @@ export namespace Crafter {
                                     std::array<float,4> clipRectPx = {0.0f, 0.0f, 1e9f, 1e9f},
                                     std::uint32_t flags = 0) const noexcept;
 
+        // Builds the header `flags` word: the caller's user flags with the
+        // reserved kUIFlagClip bit set iff `clipRectPx` does not already cover
+        // the whole surface. When the clip rect spans the surface, the four
+        // per-pixel clip compares in uiResolveScreenPixel can never reject an
+        // in-surface pixel, so the bit stays clear and the shaders skip them.
+        // Static + dimension-parameterised so the decision is unit-testable
+        // without a live Window; FillHeader feeds it the current window size.
+        static std::uint32_t ClipFlags(std::array<float,4> clipRectPx,
+                                       std::uint32_t surfaceWidth,
+                                       std::uint32_t surfaceHeight,
+                                       std::uint32_t flags) noexcept;
+
         void DispatchQuads(GraphicsCommandBuffer cmd, std::uint32_t bufferSlot, std::uint32_t itemCount,
                            std::array<float,4> clipRectPx = {0.0f, 0.0f, 1e9f, 1e9f});
         void DispatchCircles(GraphicsCommandBuffer cmd, std::uint32_t bufferSlot, std::uint32_t itemCount,

project.cpp

@@ -442,6 +442,64 @@ extern "C" Configuration CrafterBuildProject(std::span<const std::string_view> a
         advImpls.emplace_back("tests/FontAdvanceCache/main");
         vc.GetInterfacesAndImplementations(ifaces, advImpls);
         cfg.tests.push_back(std::move(advTest));
+
+        // Issue #50: uiResolveScreenPixel now gates its per-pixel clip-rect
+        // compares on the reserved kUIFlagClip bit, which FillHeader sets only
+        // when the clip rect is narrower than the surface. The decision lives
+        // in UIRenderer::ClipFlags — pure CPU logic over the clip rect and the
+        // surface size — so this test drives it directly with synthetic
+        // dimensions, no Window or GPU device needed at runtime.
+        Test clipTest;
+        Configuration& clc = clipTest.config;
+        clc.path                 = cfg.path;
+        clc.name                 = "UIClipFlag";
+        clc.outputName           = "UIClipFlag";
+        clc.type                 = ConfigurationType::Executable;
+        clc.target               = cfg.target;
+        clc.march                = cfg.march;
+        clc.mtune                = cfg.mtune;
+        clc.debug                = cfg.debug;
+        clc.sysroot              = cfg.sysroot;
+        clc.dependencies         = cfg.dependencies;
+        clc.externalDependencies = cfg.externalDependencies;
+        clc.compileFlags         = cfg.compileFlags;
+        clc.linkFlags            = cfg.linkFlags;
+        clc.defines              = cfg.defines;
+        clc.cFiles               = cfg.cFiles;
+        std::vector<fs::path> clipImpls(impls.begin(), impls.end());
+        clipImpls.emplace_back("tests/UIClipFlag/main");
+        clc.GetInterfacesAndImplementations(ifaces, clipImpls);
+        cfg.tests.push_back(std::move(clipTest));
+
+        // Issue #56: InputField_HitTestCursor mapped a click x to a cursor byte
+        // offset by re-walking the prefix for every boundary (O(n^2) glyph
+        // metric lookups) over raw byte boundaries. It now delegates to
+        // Font::NearestCursorByte — one cumulative-advance pass over codepoint
+        // boundaries. The mapping is pure CPU over a TrueType file, so this test
+        // drives it directly: no Vulkan device or window. The font is copied
+        // next to the binary; the test also probes the project-root path.
+        Test hitTest;
+        Configuration& hc = hitTest.config;
+        hc.path                 = cfg.path;
+        hc.name                 = "InputFieldHitTest";
+        hc.outputName           = "InputFieldHitTest";
+        hc.type                 = ConfigurationType::Executable;
+        hc.target               = cfg.target;
+        hc.march                = cfg.march;
+        hc.mtune                = cfg.mtune;
+        hc.debug                = cfg.debug;
+        hc.sysroot              = cfg.sysroot;
+        hc.dependencies         = cfg.dependencies;
+        hc.externalDependencies = cfg.externalDependencies;
+        hc.compileFlags         = cfg.compileFlags;
+        hc.linkFlags            = cfg.linkFlags;
+        hc.defines              = cfg.defines;
+        hc.cFiles               = cfg.cFiles;
+        hc.files                = { fs::path("tests/InputFieldHitTest/font.ttf") };
+        std::vector<fs::path> hitImpls(impls.begin(), impls.end());
+        hitImpls.emplace_back("tests/InputFieldHitTest/main");
+        hc.GetInterfacesAndImplementations(ifaces, hitImpls);
+        cfg.tests.push_back(std::move(hitTest));
     }
 
     return cfg;

shaders/ui-shared.glsl

@@ -27,10 +27,19 @@ struct UIDispatchHeader {
     vec4  clipRectPx;     // (xy, wh) — every standard shader honors this
     uint  itemCount;
     uint  frameIdx;
-    uint  flags;          // user-defined feature bits
+    uint  flags;          // feature bits — high bit reserved by the renderer
     uint  _pad;           // reserved — keep zeroed
 };
 
+// ─── header flag bits ───────────────────────────────────────────────────
+// `flags` is otherwise free for user-defined feature bits, but the renderer
+// reserves the high bit: UI_FLAG_CLIP is set by UIRenderer::FillHeader when
+// the clip rect does not already cover the whole surface. uiResolveScreenPixel
+// gates its four clipRectPx compares on this bit so the overwhelmingly common
+// full-surface case (clipRect == {0,0,1e9,1e9}) skips them entirely. The
+// branch is push-constant-uniform across the dispatch, so it never diverges.
+const uint UI_FLAG_CLIP = 0x80000000u;
+
 // ─── standard item structs ──────────────────────────────────────────────
 // These match the C++ Crafter::QuadItem / CircleItem / ImageItem / GlyphItem
 // byte-for-byte under std430.
@@ -121,9 +130,11 @@ QuadItem LoadQuadItem(uint heap, uint i) {
 bool uiResolveScreenPixel(UIDispatchHeader hdr, out ivec2 screenPx) {
     uvec2 px = gl_GlobalInvocationID.xy;
     if (px.x >= hdr.surfaceSize.x || px.y >= hdr.surfaceSize.y) return false;
+    if ((hdr.flags & UI_FLAG_CLIP) != 0u) {
         if (float(px.x) < hdr.clipRectPx.x || float(px.y) < hdr.clipRectPx.y) return false;
         if (float(px.x) >= hdr.clipRectPx.x + hdr.clipRectPx.z) return false;
         if (float(px.y) >= hdr.clipRectPx.y + hdr.clipRectPx.w) return false;
+    }
     screenPx = ivec2(px);
     return true;
 }

tests/InputFieldHitTest/main.cpp

@@ -0,0 +1,212 @@
+/*
+Crafter®.Graphics
+Copyright (C) 2026 Catcrafts®
+catcrafts.net
+
+This library is free software; you can redistribute it and/or
+modify it under the terms of the GNU Lesser General Public
+License version 3.0 as published by the Free Software Foundation;
+
+This library is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+Lesser General Public License for more details.
+
+You should have received a copy of the GNU Lesser General Public
+License along with this library; if not, write to the Free Software
+Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301 USA
+*/
+
+// Regression test for issue #56: InputField_HitTestCursor used to map a click
+// x-coord to a cursor byte offset by calling Font::GetLineWidth on every
+// prefix `value[0..i]` for i = 1..size — each call re-walks from byte 0, so it
+// was O(n^2) glyph-metric lookups. Worse, it iterated raw byte boundaries, so
+// on multibyte UTF-8 input it could return an offset that splits a codepoint
+// (an invalid cursor position, since cursorPos is a byte offset edited via
+// value.erase(cursorPos, 1)).
+//
+// The hit test now delegates to Font::NearestCursorByte: one left-to-right
+// cumulative-advance pass (O(n) metrics) over codepoint boundaries, returning
+// the byte offset of the boundary whose cumulative advance is nearest the
+// target. This test pins:
+//   - the result matches an independent brute-force "nearest codepoint
+//     boundary" reference (built from per-prefix GetLineWidth) for many click
+//     positions across ASCII and multibyte strings,
+//   - clicks at/left of the text start return 0, clicks far past the end
+//     return value.size() (the end-of-text caret),
+//   - every returned offset lands on a codepoint boundary (never mid-sequence),
+//   - clicking exactly on a glyph's left edge lands on that glyph's boundary.
+//
+// Pure CPU + a TrueType file — no Vulkan device or window needed. The font is
+// copied next to the binary; the test also probes the project-root path.
+
+#include <cstdlib>
+
+import Crafter.Graphics;
+import std;
+
+using namespace Crafter;
+
+namespace {
+
+int failures = 0;
+
+void Check(bool ok, std::string_view what) {
+    std::println("{}  {}", ok ? "PASS" : "FAIL", what);
+    if (!ok) ++failures;
+}
+
+std::filesystem::path FindFont() {
+    for (const char* cand : {
+            "font.ttf",
+            "tests/InputFieldHitTest/font.ttf",
+            "../../examples/HelloUI/font.ttf" }) {
+        if (std::filesystem::exists(cand)) return cand;
+    }
+    return "font.ttf";
+}
+
+// All codepoint-boundary byte offsets of `s`, including 0 and s.size().
+std::vector<std::size_t> Boundaries(std::string_view s) {
+    std::vector<std::size_t> b{0};
+    std::size_t i = 0;
+    while (i < s.size()) {
+        std::uint32_t cp = DecodeUtf8(s, i);   // advances i to the next boundary
+        if (cp == 0) break;
+        b.push_back(i);
+    }
+    return b;
+}
+
+// Independent reference: the codepoint boundary whose prefix advance is
+// nearest `target`, ties resolved to the earlier boundary (matching the
+// implementation's strict `d < bestDist`). Built with per-prefix GetLineWidth
+// — the slow path the optimisation replaces, used here only as the oracle.
+std::size_t NearestBoundaryRef(Font& font, std::string_view s, float size, float target) {
+    if (target <= 0.0f) return 0;
+    auto bounds = Boundaries(s);
+    std::size_t best = 0;
+    float bestDist = std::abs(target);   // boundary 0 has width 0
+    for (std::size_t k = 1; k < bounds.size(); ++k) {
+        float w = static_cast<float>(font.GetLineWidth(s.substr(0, bounds[k]), size));
+        float d = std::abs(target - w);
+        if (d < bestDist) {
+            bestDist = d;
+            best = bounds[k];
+        }
+        // No early break: the reference scans every boundary regardless, so a
+        // mismatch would also catch any over-eager break in the implementation.
+    }
+    return best;
+}
+
+bool IsBoundary(std::string_view s, std::size_t off) {
+    if (off > s.size()) return false;
+    for (std::size_t b : Boundaries(s)) if (b == off) return true;
+    return false;
+}
+
+const InputFieldColors kColors{
+    {0, 0, 0, 1}, {0, 0, 0, 1}, {0, 0, 0, 1}, {0, 0, 0, 1},
+    {1, 1, 1, 1}, {1, 1, 1, 1},
+};
+
+// Drive the public hit-test for a click at window-x `clickX`, given a field
+// rect at `rectX`. paddingX is taken from kColors.
+std::size_t Hit(const InputField& f, Font& font, float size, float rectX, float clickX) {
+    Rect rect{rectX, 0.0f, 200.0f, 24.0f};
+    return InputField_HitTestCursor(f, rect, clickX, font, size, kColors);
+}
+
+} // namespace
+
+int main() {
+    Font font(FindFont());
+
+    constexpr float kSize = 18.0f;
+    const float rectX = 30.0f;
+    const float textStart = rectX + kColors.paddingX;   // x where the text begins
+
+    const std::array<std::string_view, 4> samples = {
+        "Hello, world",
+        "1234567890",
+        "iiiiWWWWmmmm",          // wildly varying advances
+        "caf\xC3\xA9 na\xC3\xAFve \xE2\x9C\x93",   // "café naïve ✓" — 2- and 3-byte cps
+    };
+
+    // 1. Across each sample, sweep target px and compare to the oracle.
+    bool sweepOk = true;
+    bool allBoundaries = true;
+    for (std::string_view s : samples) {
+        InputField f;
+        f.type  = InputFieldType::Text;
+        f.value = std::string(s);
+        float fullW = static_cast<float>(font.GetLineWidth(s, kSize));
+        // Step finely enough to cross every glyph edge several times, and run
+        // past the end of the text to exercise the end-of-text caret.
+        for (float t = -10.0f; t <= fullW + 40.0f; t += 1.0f) {
+            std::size_t got = Hit(f, font, kSize, rectX, textStart + t);
+            std::size_t ref = NearestBoundaryRef(font, s, kSize, t);
+            if (got != ref) {
+                sweepOk = false;
+                std::println("  mismatch on \"{}\": target={} got={} ref={}",
+                             s, t, got, ref);
+            }
+            if (!IsBoundary(s, got)) allBoundaries = false;
+        }
+    }
+    Check(sweepOk, "hit test matches the brute-force nearest-boundary oracle across a px sweep");
+    Check(allBoundaries, "every returned offset lands on a UTF-8 codepoint boundary");
+
+    // 2. Clicks at or left of the text start return 0.
+    {
+        InputField f; f.value = "Hello";
+        Check(Hit(f, font, kSize, rectX, textStart) == 0,
+              "click exactly at the text start returns offset 0");
+        Check(Hit(f, font, kSize, rectX, textStart - 5.0f) == 0,
+              "click left of the text start returns offset 0");
+        Check(Hit(f, font, kSize, rectX, 0.0f) == 0,
+              "click far to the left of the field returns offset 0");
+    }
+
+    // 3. A click far past the end lands on the end-of-text caret (value.size()).
+    {
+        InputField f; f.value = "Hello";
+        float fullW = static_cast<float>(font.GetLineWidth(f.value, kSize));
+        Check(Hit(f, font, kSize, rectX, textStart + fullW + 1000.0f) == f.value.size(),
+              "click far past the end returns value.size()");
+    }
+
+    // 4. An empty field always resolves to offset 0.
+    {
+        InputField f; f.value = "";
+        Check(Hit(f, font, kSize, rectX, textStart + 50.0f) == 0,
+              "empty field returns offset 0 regardless of click x");
+    }
+
+    // 5. Clicking just past a glyph's right edge selects that glyph's boundary
+    //    (cursor sits after the glyph). Walk the boundaries of an ASCII string
+    //    and click slightly past each cumulative advance.
+    {
+        std::string_view s = "abcXYZ";
+        InputField f; f.value = std::string(s);
+        auto bounds = Boundaries(s);
+        bool edgeOk = true;
+        for (std::size_t k = 1; k < bounds.size(); ++k) {
+            float w = static_cast<float>(font.GetLineWidth(s.substr(0, bounds[k]), kSize));
+            // Click a hair before the glyph's right edge → nearest boundary is
+            // this one (its advance is the closest of all boundaries).
+            std::size_t got = Hit(f, font, kSize, rectX, textStart + w - 0.5f);
+            if (got != bounds[k]) {
+                edgeOk = false;
+                std::println("  edge mismatch at boundary {} (w={}): got {}",
+                             bounds[k], w, got);
+            }
+        }
+        Check(edgeOk, "clicking near a glyph's right edge resolves to that glyph's boundary");
+    }
+
+    if (failures == 0) std::println("\nAll InputField hit-test checks passed.");
+    else               std::println("\n{} InputField hit-test check(s) FAILED.", failures);
+    return failures == 0 ? EXIT_SUCCESS : EXIT_FAILURE;
+}

tests/UIClipFlag/main.cpp

@@ -0,0 +1,105 @@
+/*
+Crafter®.Graphics
+Copyright (C) 2026 Catcrafts®
+catcrafts.net
+
+This library is free software; you can redistribute it and/or
+modify it under the terms of the GNU Lesser General Public
+License version 3.0 as published by the Free Software Foundation;
+
+This library is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+Lesser General Public License for more details.
+
+You should have received a copy of the GNU Lesser General Public
+License along with this library; if not, write to the Free Software
+Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301 USA
+*/
+
+// Regression test for issue #50: uiResolveScreenPixel used to run four float
+// compares against hdr.clipRectPx for every pixel of every dispatch, even
+// though the clip rect is the full-surface default {0,0,1e9,1e9} in the
+// overwhelmingly common case. The compares are now gated on the reserved
+// kUIFlagClip bit, which UIRenderer::FillHeader sets (via ClipFlags) only when
+// the clip rect is actually narrower than the surface.
+//
+// ClipFlags is the pure CPU decision behind that gate: given a clip rect, the
+// surface size, and the caller's user flags, it returns the header flag word
+// with kUIFlagClip set iff the rect does not cover the whole surface. This
+// test drives it directly with synthetic dimensions — no Window or device.
+//
+// The contract it must hold for the shader gate to be correct:
+//   * full-surface (and over-covering) clips must NOT set the bit, so the
+//     shader skips the compares — and the skipped compares would never have
+//     rejected an in-surface pixel anyway, so output is pixel-identical;
+//   * any clip narrower than the surface on ANY edge MUST set the bit;
+//   * the caller's user flag bits are preserved either way; only the reserved
+//     high bit is touched.
+
+#include <cstdlib>
+
+import Crafter.Graphics;
+import std;
+using namespace Crafter;
+
+namespace {
+
+int failures = 0;
+
+void Check(bool ok, std::string_view what) {
+    std::println("{}  {}", ok ? "PASS" : "FAIL", what);
+    if (!ok) ++failures;
+}
+
+constexpr std::array<float,4> kFullDefault = {0.0f, 0.0f, 1e9f, 1e9f};
+
+bool ClipBitSet(std::array<float,4> rect, std::uint32_t w, std::uint32_t h,
+                std::uint32_t userFlags = 0) {
+    return (UIRenderer::ClipFlags(rect, w, h, userFlags) & kUIFlagClip) != 0u;
+}
+
+} // namespace
+
+int main() {
+    constexpr std::uint32_t W = 1920, H = 1080;
+
+    // --- full-surface clips leave the bit clear (the fast path) -----------
+    Check(!ClipBitSet(kFullDefault, W, H),
+          "the {0,0,1e9,1e9} default does not set the clip bit");
+    Check(!ClipBitSet({0.0f, 0.0f, float(W), float(H)}, W, H),
+          "a clip exactly matching the surface does not set the clip bit");
+    Check(!ClipBitSet({-5.0f, -5.0f, float(W) + 10.0f, float(H) + 10.0f}, W, H),
+          "a clip that over-covers the surface does not set the clip bit");
+
+    // --- any narrower edge sets the bit -----------------------------------
+    Check(ClipBitSet({0.0f, 0.0f, float(W) - 1.0f, float(H)}, W, H),
+          "a clip one pixel short on the right edge sets the clip bit");
+    Check(ClipBitSet({0.0f, 0.0f, float(W), float(H) - 1.0f}, W, H),
+          "a clip one pixel short on the bottom edge sets the clip bit");
+    Check(ClipBitSet({1.0f, 0.0f, float(W), float(H)}, W, H),
+          "a clip inset on the left edge sets the clip bit");
+    Check(ClipBitSet({0.0f, 1.0f, float(W), float(H)}, W, H),
+          "a clip inset on the top edge sets the clip bit");
+    Check(ClipBitSet({100.0f, 100.0f, 200.0f, 200.0f}, W, H),
+          "a small interior clip rect sets the clip bit");
+
+    // --- user flag bits survive the decision ------------------------------
+    constexpr std::uint32_t kUser = 0x0000000Fu; // four low user bits
+    Check(UIRenderer::ClipFlags(kFullDefault, W, H, kUser) == kUser,
+          "full-surface clip preserves user flags and clears the reserved bit");
+    Check(UIRenderer::ClipFlags({0.0f, 0.0f, 200.0f, 200.0f}, W, H, kUser)
+              == (kUser | kUIFlagClip),
+          "narrow clip preserves user flags and sets the reserved bit");
+    // A caller that (wrongly) pre-set the reserved bit must not flip the
+    // decision for a full-surface clip — ClipFlags owns that bit.
+    Check(UIRenderer::ClipFlags(kFullDefault, W, H, kUIFlagClip | kUser) == kUser,
+          "full-surface clip clears a stale reserved bit the caller passed in");
+
+    if (failures != 0) {
+        std::println(std::cerr, "{} case(s) failed", failures);
+        return 1;
+    }
+    std::println(std::cout, "all UI clip-flag cases passed");
+    return 0;
+}