diff --git a/GUIDELINES.md b/GUIDELINES.md
index aaab68d..942b4dc 100644
--- a/GUIDELINES.md
+++ b/GUIDELINES.md
@@ -533,6 +533,46 @@ tune. This is a first-class goal, bounded by sections 3.1 and 11.
 5. **One listener, one write per frame** for pointer-driven effects.
 6. **ES5-compatible style** in `niveus.js`, to match the file.
 
+### 14.4 Cross-engine rules
+
+Each of these was a real defect, found by driving Firefox with real input.
+
+1. **Never wrap an appearance change in a page-wide view transition.** The
+   snapshot cannot blur what sits behind frosted glass, so for its whole
+   duration every frosted bar, menu and Glaze pane goes clear and shows the page
+   sharply through itself. Let surfaces transition their own colours. Selection
+   markers still use a short view transition; keep it brief, and prefer a
+   per-element animation for anything inside frosted chrome.
+2. **Do not use padding to make a scroll container's last item reachable.**
+   Firefox leaves the container's bottom padding out of its scrollable area, so
+   the final rows were unreachable and a short list could not scroll at all. Use
+   real boxes — `::before` / `::after` spacers — which every engine counts.
+3. **Normalise wheel deltas.** Chromium reports a notch in pixels (~100);
+   Firefox reports it in lines (~3). Convert with `deltaMode` before using
+   `deltaY`, or an effect that works in one engine is invisible in the other.
+4. **A scrollable list is not draggable on desktop.** Touch scrolls natively; a
+   mouse does not. Anything that is meant to feel like a phone control (a wheel,
+   a carousel) needs pointer-drag handling, with inertia, an edge that gives, and
+   a click that still works when the pointer did not move.
+5. **Shadows need something to darken.** A black shadow on a near-black canvas
+   is invisible, however correct its computed value. The dark canvas is
+   deliberately lighter than the shadow colour, and soft layers use only a mild
+   negative spread so the blur reaches outside the element.
+6. **Where a CSS feature is missing, supply the same curve in script** (scroll-
+   driven animation and `interpolate-size` are absent in Firefox), and keep the
+   plain state correct without either.
+7. **State that the browser only half-supports needs an attribute.** A checkbox
+   can *show* a partial state but a click never *chooses* it;
+   `data-nv-tristate` makes the cycle explicit.
+8. **Same-page links travel.** `scroll-behavior: smooth` under
+   `prefers-reduced-motion: no-preference`; a jump to a section is the textbook
+   change a user could have watched.
+9. **Know your test rig.** A headless browser without a GPU does not render
+   `backdrop-filter`, reports no hover capability (so hover-only controls such
+   as scroller arrows disappear), and may not deliver scroll events reliably.
+   Treat those as limits of the rig, not of the system, and confirm them on a
+   real display.
+
 ### 14.3 Documentation
 
 Every new class, token, attribute, function or event is added to `REFERENCE.md`
@@ -604,9 +644,13 @@ Niveus code, follow it literally.
    python3 tools/build.py --check  # dist/ matches sources
    ```
 
-7. **Look at it.** Render both appearances in a real browser at a realistic size
-   and inspect the result. Passing checks prove the rules; they do not prove it
-   looks right or works. Exercise interactive components, not just their markup.
+7. **Look at it, in Firefox first.** Chromium implements nearly everything
+   Niveus uses, so passing there proves little; Firefox is the stricter test and
+   is where regressions have actually appeared. Render both appearances at a
+   realistic size and exercise the interactive parts with *real input* — pointer
+   drags, wheel notches, key presses — not only synthetic events or static
+   screenshots. Passing checks prove the rules; they do not prove it looks right
+   or works. Then confirm in Chromium. See 14.4 for what differs.
 8. **Report faithfully.** State what you verified, what you could not, and what
    failed. A test that failed because of the test harness is still reported.
 
@@ -651,6 +695,7 @@ A change is done when:
 - [ ] It is documented in `REFERENCE.md`; `README.md` and this document are
       updated if a principle or the component list changed.
 - [ ] It renders correctly in **light and dark**, with and without `niveus.js`.
+- [ ] It was seen and exercised in **Firefox** with real input, then in Chromium.
 - [ ] Every interaction state exists and is reachable by keyboard.
 - [ ] Translucent surfaces have a solid fallback.
 - [ ] It respects reduced motion and reduced transparency.
@@ -714,7 +759,7 @@ Use this for review, human or automated. Every line is a yes/no.
 - [ ] No `!important`; application CSS is unlayered.
 - [ ] `REFERENCE.md` updated; `dist/` rebuilt, not hand-edited.
 - [ ] `build.py`, `check.py`, `build.py --check` pass.
-- [ ] Seen in a browser; interactive parts exercised.
+- [ ] Seen and exercised in Firefox with real input, then Chromium.
 
 ---
 
diff --git a/REFERENCE.md b/REFERENCE.md
index 28ae7b4..89da370 100644
--- a/REFERENCE.md
+++ b/REFERENCE.md
@@ -527,6 +527,7 @@ everything still renders — state changes simply arrive instead of travelling.
 | `data-nv-calendar-step` | button in a calendar | `-1` or `1`: months to move. |
 | `data-nv-timepicker` | `.nv-timepicker` | Marks a time picker. Also `data-nv-value` (`HH:MM`), `-hours` (`24` default / `12`), `-step` (minutes), `-name` (hidden form field), `-format-toggle` (adds the 24H / AM / PM wheel). |
 | `data-nv-picker-label` | child of a picker trigger | Receives the chosen option's text; default is the trigger itself. |
+| `data-nv-tristate` | `input[type=checkbox]` | Click cycles unchecked → checked → partial. `="partial"` starts partial. Exposes `data-nv-state` (`off`/`on`/`partial`). |
 | `data-nv-scroller` | `.nv-scroller-frame` | Wires its `[data-nv-scroll="prev"\|"next"]` controls. |
 | `data-scrolled` | `.nv-nav` | Set by the application on scroll; gives the bar its shadow. |
 | `data-nv-direction` | `<html>` | Set during a transition by `nv.transition`. |
diff --git a/dist/niveus.css b/dist/niveus.css
index e18d6e0..b85cacc 100644
--- a/dist/niveus.css
+++ b/dist/niveus.css
@@ -527,6 +527,12 @@
      consistent between appearances: an ambient contact shadow that keeps the
      edge crisp, a soft penumbra for the spread, and an umbra that only appears
      once a surface is genuinely far from the canvas.
+
+     Two things keep a shadow visible, and both have failed before. The soft
+     layers use only a mild negative spread: a deep one tucks the blur under the
+     element and leaves almost nothing outside it. And the canvas has to be
+     lighter than the shadow colour — black cannot darken black — which is why the
+     dark canvas is not the darkest value it could be.
      ========================================================================== */
 
   :root {
@@ -535,27 +541,27 @@
     --nv-elevation-1:
       0 1px 1px var(--nv-color-shadow-ambient),
       0 2px 4px -1px var(--nv-color-shadow-penumbra),
-      0 6px 14px -8px var(--nv-color-shadow-umbra);
+      0 6px 14px -5px var(--nv-color-shadow-umbra);
 
     --nv-elevation-2:
       0 1px 1px var(--nv-color-shadow-ambient),
       0 3px 6px -2px var(--nv-color-shadow-penumbra),
-      0 10px 20px -10px var(--nv-color-shadow-umbra);
+      0 10px 20px -7px var(--nv-color-shadow-umbra);
 
     --nv-elevation-3:
       0 1px 2px var(--nv-color-shadow-ambient),
       0 6px 12px -4px var(--nv-color-shadow-penumbra),
-      0 18px 36px -12px var(--nv-color-shadow-umbra);
+      0 18px 36px -9px var(--nv-color-shadow-umbra);
 
     --nv-elevation-4:
       0 1px 2px var(--nv-color-shadow-ambient),
       0 10px 20px -6px var(--nv-color-shadow-penumbra),
-      0 30px 60px -16px var(--nv-color-shadow-umbra);
+      0 30px 60px -12px var(--nv-color-shadow-umbra);
 
     --nv-elevation-5:
       0 2px 3px var(--nv-color-shadow-ambient),
       0 16px 32px -8px var(--nv-color-shadow-penumbra),
-      0 44px 88px -20px var(--nv-color-shadow-umbra);
+      0 44px 88px -16px var(--nv-color-shadow-umbra);
 
     /* --- Inset ---------------------------------------------------------------
        Recessed surfaces: fields, wells, tracks, pressed controls. */
@@ -1093,10 +1099,10 @@
   :root,
   [data-nv-brand] {
     /* --- Canvas and surfaces ----------------------------------------------- */
-    --nv-dark-background: color-mix(in oklab, #0f0f12, var(--nv-brand) var(--nv-brand-tint));
-    --nv-dark-background-subtle: color-mix(in oklab, #18181b, var(--nv-brand) var(--nv-brand-tint));
-    --nv-dark-surface: color-mix(in oklab, #1f1f22, var(--nv-brand) var(--nv-brand-tint));
-    --nv-dark-surface-elevated: color-mix(in oklab, #2a2a2e, var(--nv-brand) var(--nv-brand-tint));
+    --nv-dark-background: color-mix(in oklab, #17171b, var(--nv-brand) var(--nv-brand-tint));
+    --nv-dark-background-subtle: color-mix(in oklab, #1d1d21, var(--nv-brand) var(--nv-brand-tint));
+    --nv-dark-surface: color-mix(in oklab, #232327, var(--nv-brand) var(--nv-brand-tint));
+    --nv-dark-surface-elevated: color-mix(in oklab, #2d2d32, var(--nv-brand) var(--nv-brand-tint));
     --nv-dark-surface-inset: color-mix(in oklab, #111113, var(--nv-brand) var(--nv-brand-tint));
     --nv-dark-surface-hover: color-mix(in oklab, #313136, var(--nv-brand) var(--nv-brand-tint));
     --nv-dark-surface-active: color-mix(in oklab, #39393f, var(--nv-brand) var(--nv-brand-tint));
@@ -1183,9 +1189,9 @@
       calc(c * 0.2) h);
 
     /* --- Depth and material -------------------------------------------------- */
-    --nv-dark-shadow-ambient: rgb(0 0 0 / 0.5);
-    --nv-dark-shadow-penumbra: rgb(0 0 0 / 0.5);
-    --nv-dark-shadow-umbra: rgb(0 0 0 / 0.55);
+    --nv-dark-shadow-ambient: rgb(0 0 0 / 0.6);
+    --nv-dark-shadow-penumbra: rgb(0 0 0 / 0.62);
+    --nv-dark-shadow-umbra: rgb(0 0 0 / 0.72);
     --nv-dark-shadow-inset: rgb(0 0 0 / 0.65);
     --nv-dark-highlight: rgb(255 255 255 / 0.15);
     --nv-dark-highlight-strong: rgb(255 255 255 / 0.24);
@@ -1374,6 +1380,13 @@
     border-radius: var(--nv-radius-xs);
   }
 
+  /* A link to a place on the same page travels there instead of cutting. */
+  @media (prefers-reduced-motion: no-preference) {
+    html {
+      scroll-behavior: smooth;
+    }
+  }
+
   /* Whatever the browser draws itself — a bare checkbox, a progress bar, the
      text caret, a date field's glyphs — takes the brand, so no native colour
      survives next to the system's own. */
@@ -4119,8 +4132,11 @@
     inline-size: 3.5rem;
     block-size: calc(var(--_row) * var(--_rows));
     margin: 0;
-    padding: calc(var(--_row) * (var(--_rows) - 1) / 2) 0;
+    padding: 0;
     list-style: none;
+    cursor: grab;
+    user-select: none;
+    touch-action: pan-y;
     overflow-y: auto;
     overscroll-behavior: contain;
     scroll-snap-type: y mandatory;
@@ -4140,6 +4156,21 @@
     display: none;
   }
 
+  /* Spacers, not padding: they let the first and last rows reach the centre.
+     Firefox leaves the bottom padding of a scroll container out of its
+     scrollable area, which stranded the last rows (and made a three-row wheel
+     unscrollable); real boxes always count. */
+  .nv-timepicker__column::before,
+  .nv-timepicker__column::after {
+    content: "";
+    display: block;
+    block-size: calc(var(--_row) * (var(--_rows) - 1) / 2);
+  }
+
+  .nv-timepicker__column[data-nv-dragging] {
+    cursor: grabbing;
+  }
+
   .nv-timepicker__column:focus-visible {
     outline: none;
     border-radius: var(--nv-radius-md);
diff --git a/index.html b/index.html
index 757cec9..db92b1d 100644
--- a/index.html
+++ b/index.html
@@ -768,7 +768,7 @@
             Weekly digest
           </label>
           <label class="nv-choice">
-            <input class="nv-checkbox" type="checkbox" id="f-indeterminate">
+            <input class="nv-checkbox" type="checkbox" id="f-indeterminate" data-nv-tristate="partial">
             Partial selection
           </label>
           <label class="nv-choice">
@@ -1527,7 +1527,6 @@
     addEventListener("scroll", onScroll, { passive: true });
     onScroll();
 
-    document.getElementById("f-indeterminate").indeterminate = true;
 
     /* Build the semantic color list from the tokens themselves, so the page
        cannot show a role the system does not actually define. */
diff --git a/niveus/base.css b/niveus/base.css
index 8f60033..01f60f8 100644
--- a/niveus/base.css
+++ b/niveus/base.css
@@ -134,6 +134,13 @@ textarea {
   border-radius: var(--nv-radius-xs);
 }
 
+/* A link to a place on the same page travels there instead of cutting. */
+@media (prefers-reduced-motion: no-preference) {
+  html {
+    scroll-behavior: smooth;
+  }
+}
+
 /* Whatever the browser draws itself — a bare checkbox, a progress bar, the
    text caret, a date field's glyphs — takes the brand, so no native colour
    survives next to the system's own. */
diff --git a/niveus/components/timepicker.css b/niveus/components/timepicker.css
index 126030e..1affb1a 100644
--- a/niveus/components/timepicker.css
+++ b/niveus/components/timepicker.css
@@ -65,8 +65,11 @@
   inline-size: 3.5rem;
   block-size: calc(var(--_row) * var(--_rows));
   margin: 0;
-  padding: calc(var(--_row) * (var(--_rows) - 1) / 2) 0;
+  padding: 0;
   list-style: none;
+  cursor: grab;
+  user-select: none;
+  touch-action: pan-y;
   overflow-y: auto;
   overscroll-behavior: contain;
   scroll-snap-type: y mandatory;
@@ -86,6 +89,21 @@
   display: none;
 }
 
+/* Spacers, not padding: they let the first and last rows reach the centre.
+   Firefox leaves the bottom padding of a scroll container out of its
+   scrollable area, which stranded the last rows (and made a three-row wheel
+   unscrollable); real boxes always count. */
+.nv-timepicker__column::before,
+.nv-timepicker__column::after {
+  content: "";
+  display: block;
+  block-size: calc(var(--_row) * (var(--_rows) - 1) / 2);
+}
+
+.nv-timepicker__column[data-nv-dragging] {
+  cursor: grabbing;
+}
+
 .nv-timepicker__column:focus-visible {
   outline: none;
   border-radius: var(--nv-radius-md);
diff --git a/niveus/niveus.js b/niveus/niveus.js
index 7e40e05..816c772 100644
--- a/niveus/niveus.js
+++ b/niveus/niveus.js
@@ -96,17 +96,19 @@
 
   /** "light" | "dark" | "system" — system means: follow the environment. */
   function setTheme(theme) {
-    var done = transition(function () {
-      if (theme === "system") {
-        root.removeAttribute("data-theme");
-      } else {
-        root.setAttribute("data-theme", theme);
-      }
-      syncThemeControls(theme);
-    }, { kind: "theme" });
-
+    // Deliberately not a view transition. A page-wide snapshot cannot blur what
+    // sits behind frosted glass, so for the length of the cross-fade every
+    // frosted bar, menu and glaze pane went clear and showed the page sharply
+    // through itself. Surfaces transition their own colours instead (see
+    // --nv-transition-surface), and the glass stays live throughout.
+    if (theme === "system") {
+      root.removeAttribute("data-theme");
+    } else {
+      root.setAttribute("data-theme", theme);
+    }
+    syncThemeControls(theme);
     root.dispatchEvent(new CustomEvent("nv:theme", { detail: { theme: theme } }));
-    return done;
+    return Promise.resolve();
   }
 
   function syncThemeControls(theme) {
@@ -798,18 +800,87 @@
       var settle = 0;
       var limit = 36;
 
-      function setPull(value) {
+      function clearPull() {
+        pull = 0;
+        delete list.dataset.nvPulling;
+        list.style.removeProperty("--nv-pull");
+      }
+
+      // `hold` keeps the pull until clearPull() — a finger or button is still
+      // down — instead of releasing it after the wheel goes quiet.
+      function setPull(value, hold) {
         pull = value;
         list.style.setProperty("--nv-pull", value.toFixed(1) + "px");
         list.dataset.nvPulling = "";
         global.clearTimeout(settle);
-        settle = global.setTimeout(function () {
-          pull = 0;
-          delete list.dataset.nvPulling;
-          list.style.removeProperty("--nv-pull");
-        }, 90);
+        if (!hold) settle = global.setTimeout(clearPull, 90);
       }
 
+      // Dragging with a mouse or pen. Touch scrolls natively; a desktop
+      // pointer has no such thing, so without this a wheel cannot be grabbed.
+      // Snapping is off while the pointer is down so the list follows it, the
+      // end gives with resistance, and on release the list coasts to the row
+      // the motion was heading for.
+      var drag = null;
+
+      list.addEventListener("pointerdown", function (event) {
+        if (event.pointerType === "touch" || event.button !== 0) return;
+        drag = {
+          id: event.pointerId, y: event.clientY, top: list.scrollTop,
+          lastY: event.clientY, lastT: event.timeStamp, velocity: 0, moved: false,
+        };
+      });
+
+      list.addEventListener("pointermove", function (event) {
+        if (!drag || event.pointerId !== drag.id) return;
+        var dy = event.clientY - drag.y;
+        if (!drag.moved) {
+          if (Math.abs(dy) < 4) return;
+          // Only now is it a drag and not a click, so only now is the pointer
+          // captured; capturing earlier would redirect the click to the list.
+          drag.moved = true;
+          list.setPointerCapture(event.pointerId);
+          list.style.scrollSnapType = "none";
+          list.dataset.nvDragging = "";
+        }
+        var elapsed = event.timeStamp - drag.lastT;
+        if (elapsed > 0) drag.velocity = (event.clientY - drag.lastY) / elapsed;
+        drag.lastY = event.clientY;
+        drag.lastT = event.timeStamp;
+
+        var max = list.scrollHeight - list.clientHeight;
+        var target = drag.top - dy;
+        if (target < 0 || target > max) {
+          list.scrollTop = target < 0 ? 0 : max;
+          var over = target < 0 ? -target : target - max;
+          if (!prefersReducedMotion()) {
+            setPull((target < 0 ? 1 : -1) * Math.min(limit, over * 0.35), true);
+          }
+        } else {
+          list.scrollTop = target;
+          if (pull) clearPull();
+        }
+      });
+
+      function endDrag(event) {
+        if (!drag || event.pointerId !== drag.id) return;
+        var moved = drag.moved;
+        var velocity = drag.velocity;
+        drag = null;
+        if (!moved) return;
+        delete list.dataset.nvDragging;
+        list.style.scrollSnapType = "";
+        clearPull();
+        // Coast: carry the release speed a little way on, then settle on a row.
+        var row = list.firstElementChild.offsetHeight;
+        var ahead = list.scrollTop - velocity * 160;
+        var index = Math.max(0, Math.min(list.children.length - 1, Math.round(ahead / row)));
+        scrollTo(list, index, true);
+      }
+
+      list.addEventListener("pointerup", endDrag);
+      list.addEventListener("pointercancel", endDrag);
+
       list.addEventListener("wheel", function (event) {
         if (prefersReducedMotion()) return;
         var atTop = list.scrollTop <= 0;
@@ -820,6 +891,7 @@
         var delta = event.deltaY * (event.deltaMode === 1 ? 16 : event.deltaMode === 2 ? list.clientHeight : 1);
         var pushing = (atTop && delta < 0) || (atBottom && delta > 0);
         if (!pushing && !pull) return;
+        if (drag && drag.moved) return;
         event.preventDefault();
         var next = pull - delta * 0.12 * (1 - Math.abs(pull) / limit);
         setPull(Math.max(-limit, Math.min(limit, next)));
@@ -1089,6 +1161,31 @@
     });
   }
 
+  /**
+   * A checkbox with data-nv-tristate cycles unchecked → checked → partial →
+   * unchecked. The browser only offers the first two by clicking, and the third
+   * only from script, so a partial state could be shown but never chosen.
+   * data-nv-tristate="partial" starts it partial.
+   */
+  function initTristate(box) {
+    if (box.dataset.nvTristateWired) return;
+    box.dataset.nvTristateWired = "";
+    var states = ["off", "on", "partial"];
+    var state = box.dataset.nvTristate === "partial" ? 2 : box.checked ? 1 : 0;
+
+    function apply() {
+      box.checked = state === 1;
+      box.indeterminate = state === 2;
+      box.dataset.nvState = states[state];
+    }
+
+    box.addEventListener("click", function () {
+      state = (state + 1) % 3;
+      apply();
+    });
+    apply();
+  }
+
   function wire(scope) {
     scope = scope || document;
 
@@ -1107,6 +1204,7 @@
     scope.querySelectorAll("[data-nv-tabs]").forEach(initTabs);
     scope.querySelectorAll("[data-nv-scroller]").forEach(initScroller);
     scope.querySelectorAll(".nv-slider").forEach(initSlider);
+    scope.querySelectorAll("input[data-nv-tristate]").forEach(initTristate);
     scope.querySelectorAll("[data-nv-calendar]").forEach(initCalendar);
     scope.querySelectorAll("[data-nv-timepicker]").forEach(initTimePicker);
     scope.querySelectorAll(".nv-menu--listbox").forEach(initListbox);
diff --git a/niveus/themes/dark.css b/niveus/themes/dark.css
index 30a0782..b0f161a 100644
--- a/niveus/themes/dark.css
+++ b/niveus/themes/dark.css
@@ -21,10 +21,10 @@
 :root,
 [data-nv-brand] {
   /* --- Canvas and surfaces ----------------------------------------------- */
-  --nv-dark-background: color-mix(in oklab, #0f0f12, var(--nv-brand) var(--nv-brand-tint));
-  --nv-dark-background-subtle: color-mix(in oklab, #18181b, var(--nv-brand) var(--nv-brand-tint));
-  --nv-dark-surface: color-mix(in oklab, #1f1f22, var(--nv-brand) var(--nv-brand-tint));
-  --nv-dark-surface-elevated: color-mix(in oklab, #2a2a2e, var(--nv-brand) var(--nv-brand-tint));
+  --nv-dark-background: color-mix(in oklab, #17171b, var(--nv-brand) var(--nv-brand-tint));
+  --nv-dark-background-subtle: color-mix(in oklab, #1d1d21, var(--nv-brand) var(--nv-brand-tint));
+  --nv-dark-surface: color-mix(in oklab, #232327, var(--nv-brand) var(--nv-brand-tint));
+  --nv-dark-surface-elevated: color-mix(in oklab, #2d2d32, var(--nv-brand) var(--nv-brand-tint));
   --nv-dark-surface-inset: color-mix(in oklab, #111113, var(--nv-brand) var(--nv-brand-tint));
   --nv-dark-surface-hover: color-mix(in oklab, #313136, var(--nv-brand) var(--nv-brand-tint));
   --nv-dark-surface-active: color-mix(in oklab, #39393f, var(--nv-brand) var(--nv-brand-tint));
@@ -111,9 +111,9 @@
     calc(c * 0.2) h);
 
   /* --- Depth and material -------------------------------------------------- */
-  --nv-dark-shadow-ambient: rgb(0 0 0 / 0.5);
-  --nv-dark-shadow-penumbra: rgb(0 0 0 / 0.5);
-  --nv-dark-shadow-umbra: rgb(0 0 0 / 0.55);
+  --nv-dark-shadow-ambient: rgb(0 0 0 / 0.6);
+  --nv-dark-shadow-penumbra: rgb(0 0 0 / 0.62);
+  --nv-dark-shadow-umbra: rgb(0 0 0 / 0.72);
   --nv-dark-shadow-inset: rgb(0 0 0 / 0.65);
   --nv-dark-highlight: rgb(255 255 255 / 0.15);
   --nv-dark-highlight-strong: rgb(255 255 255 / 0.24);
diff --git a/niveus/tokens/elevation.css b/niveus/tokens/elevation.css
index d185b73..5a39a4c 100644
--- a/niveus/tokens/elevation.css
+++ b/niveus/tokens/elevation.css
@@ -15,6 +15,12 @@
    consistent between appearances: an ambient contact shadow that keeps the
    edge crisp, a soft penumbra for the spread, and an umbra that only appears
    once a surface is genuinely far from the canvas.
+
+   Two things keep a shadow visible, and both have failed before. The soft
+   layers use only a mild negative spread: a deep one tucks the blur under the
+   element and leaves almost nothing outside it. And the canvas has to be
+   lighter than the shadow colour — black cannot darken black — which is why the
+   dark canvas is not the darkest value it could be.
    ========================================================================== */
 
 :root {
@@ -23,27 +29,27 @@
   --nv-elevation-1:
     0 1px 1px var(--nv-color-shadow-ambient),
     0 2px 4px -1px var(--nv-color-shadow-penumbra),
-    0 6px 14px -8px var(--nv-color-shadow-umbra);
+    0 6px 14px -5px var(--nv-color-shadow-umbra);
 
   --nv-elevation-2:
     0 1px 1px var(--nv-color-shadow-ambient),
     0 3px 6px -2px var(--nv-color-shadow-penumbra),
-    0 10px 20px -10px var(--nv-color-shadow-umbra);
+    0 10px 20px -7px var(--nv-color-shadow-umbra);
 
   --nv-elevation-3:
     0 1px 2px var(--nv-color-shadow-ambient),
     0 6px 12px -4px var(--nv-color-shadow-penumbra),
-    0 18px 36px -12px var(--nv-color-shadow-umbra);
+    0 18px 36px -9px var(--nv-color-shadow-umbra);
 
   --nv-elevation-4:
     0 1px 2px var(--nv-color-shadow-ambient),
     0 10px 20px -6px var(--nv-color-shadow-penumbra),
-    0 30px 60px -16px var(--nv-color-shadow-umbra);
+    0 30px 60px -12px var(--nv-color-shadow-umbra);
 
   --nv-elevation-5:
     0 2px 3px var(--nv-color-shadow-ambient),
     0 16px 32px -8px var(--nv-color-shadow-penumbra),
-    0 44px 88px -20px var(--nv-color-shadow-umbra);
+    0 44px 88px -16px var(--nv-color-shadow-umbra);
 
   /* --- Inset ---------------------------------------------------------------
      Recessed surfaces: fields, wells, tracks, pressed controls. */