diff --git a/src/app/coach/conversations/page.tsx b/src/app/coach/conversations/page.tsx
index 32f0e0ce8..c12927c9c 100644
--- a/src/app/coach/conversations/page.tsx
+++ b/src/app/coach/conversations/page.tsx
@@ -331,7 +331,7 @@ export default function CoachConversationsPage() {
       // ~50px on notched devices; restored to match exactly. v1.42 — the
       // shell now takes that inset above the top bar at every width, so
       // the desktop height subtracts it too, and `/coach/plans` matches.
-      className="bg-background -mx-4 -mt-6 -mb-20 flex h-[calc(100dvh-8rem-env(safe-area-inset-top,0px)-env(safe-area-inset-bottom,0px))] min-h-[32rem] flex-col overflow-hidden md:-mx-6 md:h-[calc(100dvh-4rem-env(safe-area-inset-top,0px))]"
+      className="bg-background shell-desktop:h-[calc(100dvh-4rem-env(safe-area-inset-top,0px))] -mx-4 -mt-6 -mb-20 flex h-[calc(100dvh-8rem-env(safe-area-inset-top,0px)-env(safe-area-inset-bottom,0px))] min-h-[32rem] flex-col overflow-hidden md:-mx-6"
     >
       <CoachConversationsBody readOnly={!coach.available} />
     </div>
diff --git a/src/app/coach/plans/page.tsx b/src/app/coach/plans/page.tsx
index b02fddcdc..4df3e06b7 100644
--- a/src/app/coach/plans/page.tsx
+++ b/src/app/coach/plans/page.tsx
@@ -296,7 +296,7 @@ export default function CoachPlansPage() {
       // mobile-only BottomNav band). The status-bar inset of an installed
       // app comes off as well, at every width: the shell takes it above
       // the top bar (`shell-safe-area`).
-      className="bg-background -mx-4 -mt-6 -mb-20 flex h-[calc(100dvh-8rem-env(safe-area-inset-top,0px)-env(safe-area-inset-bottom,0px))] min-h-[32rem] flex-col overflow-hidden md:-mx-6 md:h-[calc(100dvh-4rem-env(safe-area-inset-top,0px))]"
+      className="bg-background shell-desktop:h-[calc(100dvh-4rem-env(safe-area-inset-top,0px))] -mx-4 -mt-6 -mb-20 flex h-[calc(100dvh-8rem-env(safe-area-inset-top,0px)-env(safe-area-inset-bottom,0px))] min-h-[32rem] flex-col overflow-hidden md:-mx-6"
     >
       <CoachPlansBody readOnly={!coach.available} />
     </div>
diff --git a/src/app/globals.css b/src/app/globals.css
index 63361cc9f..81e798468 100644
--- a/src/app/globals.css
+++ b/src/app/globals.css
@@ -58,6 +58,30 @@
   }
 }
 
+/*
+ * `shell-desktop:` / `shell-mobile:` — which app shell the viewport gets.
+ * The desktop rail (sidebar, no bottom nav) from `md` up, EXCEPT a touch
+ * screen at most 520 px tall: a phone held sideways is 844–932 px wide but
+ * only 390–430 px tall, and the rail plus the top bar left it a sliver of
+ * content and no thumb-reachable navigation. It keeps the phone shell (top
+ * bar, bottom nav, capture button) instead. Everything that switches
+ * between the two shells keys on these, never on a bare `md:`, so the bars
+ * cannot disagree about which shell is showing.
+ */
+@custom-variant shell-desktop {
+  @media (min-width: 48rem) and (pointer: fine),
+    (min-width: 48rem) and (pointer: none),
+    (min-width: 48rem) and (min-height: 521px) {
+    @slot;
+  }
+}
+
+@custom-variant shell-mobile {
+  @media (max-width: 47.98rem), (pointer: coarse) and (max-height: 520px) {
+    @slot;
+  }
+}
+
 @theme inline {
   /* Extra-small breakpoint (30rem / 480px) for the narrowest phones — the
      default scale starts at sm (640px), which leaves a gap between an iPhone SE
diff --git a/src/components/data-list/selection-action-bar.tsx b/src/components/data-list/selection-action-bar.tsx
index 5a275913a..f7bf6a1e2 100644
--- a/src/components/data-list/selection-action-bar.tsx
+++ b/src/components/data-list/selection-action-bar.tsx
@@ -82,7 +82,7 @@ export function SelectionActionBar({
       // an `sm:` drop sat the bar behind the still-visible nav 640-767px.
       // z-30 keeps the bar above page content without needing to beat the
       // (now-cleared) nav's z-50.
-      className="bg-card border-border sticky bottom-[calc(env(safe-area-inset-bottom,0px)+5rem)] z-30 mt-2 flex items-center justify-between gap-2 rounded-lg border p-2 shadow-lg md:bottom-4"
+      className="bg-card border-border sticky bottom-[calc(env(safe-area-inset-bottom,0px)+5rem)] z-30 mt-2 flex items-center justify-between gap-2 rounded-lg border p-2 shadow-lg shell-desktop:bottom-4"
     >
       <div className="flex items-center gap-1">
         <Button
diff --git a/src/components/insights/__tests__/layout-coach-fab.test.tsx b/src/components/insights/__tests__/layout-coach-fab.test.tsx
index 32be51f50..089094891 100644
--- a/src/components/insights/__tests__/layout-coach-fab.test.tsx
+++ b/src/components/insights/__tests__/layout-coach-fab.test.tsx
@@ -81,8 +81,10 @@ describe("<LayoutCoachFab>", () => {
     expect(html).toContain(
       "bottom-[calc(env(safe-area-inset-bottom,0px)+5rem)]",
     );
+    // The bottom offset follows the shell, not a width: a phone held
+    // sideways keeps the bottom nav, so the FAB keeps clearing it there.
     expect(html).toContain(
-      "md:bottom-[calc(env(safe-area-inset-bottom,0px)+2rem)]",
+      "shell-desktop:bottom-[calc(env(safe-area-inset-bottom,0px)+2rem)]",
     );
     expect(html).toContain('data-keyboard-hide=""');
     expect(html).not.toContain("md:bottom-6");
diff --git a/src/components/insights/layout-coach-fab.tsx b/src/components/insights/layout-coach-fab.tsx
index 5ba70ca9f..4f2bf04e7 100644
--- a/src/components/insights/layout-coach-fab.tsx
+++ b/src/components/insights/layout-coach-fab.tsx
@@ -246,7 +246,7 @@ export function LayoutCoachFab() {
           // phone held sideways (844 px, past `md`) has its notch on one
           // side, and an installed iPad its home indicator along the bottom.
           "fixed right-[calc(env(safe-area-inset-right,0px)+1.5rem)] z-40 size-14 rounded-full shadow-lg md:right-[calc(env(safe-area-inset-right,0px)+2rem)]",
-          "bottom-[calc(env(safe-area-inset-bottom,0px)+5rem)] md:bottom-[calc(env(safe-area-inset-bottom,0px)+2rem)]",
+          "bottom-[calc(env(safe-area-inset-bottom,0px)+5rem)] shell-desktop:bottom-[calc(env(safe-area-inset-bottom,0px)+2rem)]",
           // Dark glyph on the purple/pink gradient — white sat at
           // ≈2.3:1 against the gradient midpoint; the background token
           // reads ≈6.5:1.
diff --git a/src/components/layout/auth-shell.tsx b/src/components/layout/auth-shell.tsx
index 35dc5b618..913e34cc2 100644
--- a/src/components/layout/auth-shell.tsx
+++ b/src/components/layout/auth-shell.tsx
@@ -458,7 +458,7 @@ export function AuthShell({
             error after the user already filled in a form. */}
         {demoMode ? <DemoBanner /> : null}
         <MaintainershipBanner />
-        <div className="flex min-h-0 flex-1 flex-col md:flex-row">
+        <div className="shell-desktop:flex-row flex min-h-0 flex-1 flex-col">
           <SidebarNav />
           {/* `min-w-0` lets the content column shrink below its children's
               intrinsic min width (e.g. the measurements table); without it
@@ -497,7 +497,7 @@ export function AuthShell({
               // for all of them at once instead of asking each new
               // absolutely-positioned child to remember its own wrapper.
               className={cn(
-                "relative flex-1 overflow-y-auto pb-[calc(4rem+env(safe-area-inset-bottom,0px))] md:pb-0",
+                "shell-desktop:pb-0 relative flex-1 overflow-y-auto pb-[calc(4rem+env(safe-area-inset-bottom,0px))]",
                 fullBleed ? "flex flex-col" : "[scrollbar-gutter:stable]",
               )}
             >
diff --git a/src/components/layout/bottom-nav.tsx b/src/components/layout/bottom-nav.tsx
index 82470be9a..fa1a3bbf4 100644
--- a/src/components/layout/bottom-nav.tsx
+++ b/src/components/layout/bottom-nav.tsx
@@ -208,7 +208,7 @@ export function BottomNav() {
         aria-label={t("nav.mobileNavigation")}
         data-slot="bottom-nav"
         data-keyboard-hide=""
-        className="bg-card border-border fixed bottom-0 left-0 z-50 min-h-16 w-full border-t pr-[env(safe-area-inset-right)] pb-[env(safe-area-inset-bottom)] pl-[env(safe-area-inset-left)] backdrop-blur-md md:hidden"
+        className="bg-card border-border shell-desktop:hidden fixed bottom-0 left-0 z-50 min-h-16 w-full border-t pr-[env(safe-area-inset-right)] pb-[env(safe-area-inset-bottom)] pl-[env(safe-area-inset-left)] backdrop-blur-md"
       >
         <div className="mx-auto flex h-16 max-w-lg items-stretch justify-around px-1">
           {primaryLeft.map(renderPrimary)}
@@ -274,7 +274,7 @@ export function BottomNav() {
           // The sheet auto-sizes to its content; padding-bottom respects
           // the iOS safe area so the inner buttons stay reachable above
           // the home-indicator on iPhone-X-class devices.
-          className="mx-auto max-w-md rounded-t-xl pb-[calc(env(safe-area-inset-bottom)+1rem)] md:hidden"
+          className="shell-desktop:hidden mx-auto max-w-md rounded-t-xl pb-[calc(env(safe-area-inset-bottom)+1rem)]"
           data-testid="bottom-nav-more-sheet"
         >
           <SheetHeader>
diff --git a/src/components/layout/sidebar-nav.tsx b/src/components/layout/sidebar-nav.tsx
index 755adc995..88031141a 100644
--- a/src/components/layout/sidebar-nav.tsx
+++ b/src/components/layout/sidebar-nav.tsx
@@ -681,7 +681,7 @@ export function SidebarNav() {
       <aside
         aria-label={t("nav.sidebar")}
         className={cn(
-          "bg-sidebar border-sidebar-border relative hidden h-full flex-shrink-0 overflow-hidden border-r transition-[width] duration-200 motion-reduce:transition-none md:flex md:flex-col",
+          "bg-sidebar border-sidebar-border shell-desktop:flex shell-desktop:flex-col relative hidden h-full flex-shrink-0 overflow-hidden border-r transition-[width] duration-200 motion-reduce:transition-none",
           collapsed ? "w-16" : "w-64",
         )}
       >
diff --git a/src/components/layout/top-bar.tsx b/src/components/layout/top-bar.tsx
index a751fc9c1..b8f31017d 100644
--- a/src/components/layout/top-bar.tsx
+++ b/src/components/layout/top-bar.tsx
@@ -97,7 +97,10 @@ export function TopBar() {
       // inside its own 4rem, a 59 px inset left the bar a 5 px content box.
     >
       {/* Mobile logo */}
-      <Link href="/" className="flex min-h-11 items-center gap-2 md:hidden">
+      <Link
+        href="/"
+        className="shell-desktop:hidden flex min-h-11 items-center gap-2"
+      >
         <Logo className="text-primary" size={20} />
         <span className="font-bold tracking-tight">HealthLog</span>
       </Link>
@@ -105,7 +108,7 @@ export function TopBar() {
       {/* Desktop: the page's context, where a page gives one (the Coach's
           trail); otherwise an empty spacer. User controls are in the
           sidebar. */}
-      <TopBarContextOutlet className="hidden min-w-0 flex-1 items-center md:flex" />
+      <TopBarContextOutlet className="shell-desktop:flex hidden min-w-0 flex-1 items-center" />
 
       {/* Page-owned actions (the Coach's conversations toggle). `ml-auto`
           keeps them at the trailing edge, right before the mobile avatar
@@ -117,7 +120,7 @@ export function TopBar() {
       <TopBarActionsOutlet className="ml-auto flex shrink-0 items-center gap-1 empty:hidden md:-mr-3" />
 
       {/* Mobile-only auth section (desktop uses sidebar user section) */}
-      <div className="flex items-center gap-2 md:hidden">
+      <div className="shell-desktop:hidden flex items-center gap-2">
         {isLoading ? (
           <Skeleton className="bg-muted h-4 w-20 rounded" />
         ) : user ? (