fix(desktop): restore the pane tab strip's bottom rule

The strip's rule is an inset shadow painted in the container's last pixel
row, and full-height tabs covered it — so each tab read as overhanging
the bar by 1px. Inactive tabs compensated with their own border, stacking
a second translucent line that darkened the seam.

Inactive tabs now stop 1px short and draw no bottom border, leaving the
container as the sole owner of one continuous rule; the active tab keeps
full height so it alone cuts through. Hover also darkens rather than
lightens, since lightening moved a hovered tab toward the active
surface's look.

apps/desktop/src/components/ui/pane-tab.tsx

@@ -12,17 +12,23 @@ export const PANE_TAB_STRIP_LINE_RIGHT = 'shadow-[inset_-1px_0_0_var(--ui-stroke
 const TAB =
   'group/tab relative flex shrink-0 items-center border-transparent bg-(--tab-bg) text-[0.6875rem] font-medium [-webkit-app-region:no-drag]'
 
-const TAB_HORIZONTAL = 'h-full min-w-0 max-w-48 border-b not-first:border-l not-first:border-l-(--ui-stroke-quaternary)'
+// Stop 1px short: the strip's rule is an INSET shadow painted in the
+// container's own last pixel row, so a full-height tab covers it and reads as
+// overhanging the bar. The container owns the line; nothing redraws it.
+const TAB_HORIZONTAL =
+  'h-[calc(100%-1px)] min-w-0 max-w-48 not-first:border-l not-first:border-l-(--ui-stroke-quaternary)'
 
 const TAB_VERTICAL =
   'w-full max-h-48 justify-center not-first:border-t not-first:border-t-(--ui-stroke-quaternary) [writing-mode:vertical-rl]'
 
-const TAB_ACTIVE = 'text-foreground [--tab-bg:var(--pane-tab-active-bg,var(--ui-editor-surface-background))]'
+// Full height, so the active tab alone covers that row and cuts the rule.
+const TAB_ACTIVE =
+  'h-full text-foreground [--tab-bg:var(--pane-tab-active-bg,var(--ui-editor-surface-background))]'
 
-// Inactive = gutter. Hover = 4% translucent wash (VS Code/GitHub alpha hover),
-// not an opaque recolor — and never touch borders.
+// Inactive = gutter. Hover DARKENS: active is the lighter content surface, so a
+// lightening wash made the two nearly indistinguishable.
 const TAB_IDLE =
-  'text-(--ui-text-tertiary) [--tab-bg:var(--pane-tab-strip-bg,var(--theme-card-seed))] hover:shadow-[inset_0_0_0_100vmax_color-mix(in_srgb,var(--ui-base)_4%,transparent)] hover:text-(--ui-text-secondary)'
+  'text-(--ui-text-tertiary) [--tab-bg:var(--pane-tab-strip-bg,var(--theme-card-seed))] hover:shadow-[inset_0_0_0_100vmax_color-mix(in_srgb,#000_var(--ui-tab-hover-darken),transparent)] hover:text-(--ui-text-secondary)'
 
 interface PaneTabProps extends React.ComponentProps<'div'> {
   active?: boolean
@@ -66,9 +72,9 @@ export const PaneTab = React.forwardRef<HTMLDivElement, PaneTabProps>(function P
   },
   ref
 ) {
-  // Content-facing edge: horizontal cuts the bottom strip line; vertical cuts
-  // the side that faces the editor (left rail → right edge, right rail → left).
-  const edge = vertical ? (side === 'right' ? 'border-l' : 'border-r') : 'border-b'
+  // Vertical rails only. Horizontal tabs draw no bottom border — the strip owns
+  // that rule, and a per-tab border stacked a second translucent line over it.
+  const edge = vertical ? (side === 'right' ? 'border-l' : 'border-r') : undefined
 
   return (
     <div
@@ -76,7 +82,7 @@ export const PaneTab = React.forwardRef<HTMLDivElement, PaneTabProps>(function P
         TAB,
         vertical ? TAB_VERTICAL : TAB_HORIZONTAL,
         edge,
-        active ? TAB_ACTIVE : cn(TAB_IDLE, `${edge}-(--ui-stroke-tertiary)`),
+        active ? TAB_ACTIVE : cn(TAB_IDLE, edge && `${edge}-(--ui-stroke-tertiary)`),
         className
       )}
       data-active={active}

apps/desktop/src/styles.css

@@ -317,6 +317,10 @@
        :root.dark bumps the alpha since a dark card swallows shadow. Removed on
        focus (the focus border carries the state). */
     --dt-input-inset: inset 0 1px 1px color-mix(in srgb, #000 10%, transparent);
+    /* Hovered inactive tab: deepen it AWAY from the lighter active surface, so
+       hover can never be mistaken for selected. Dark mode needs more — black
+       over a dark gutter shows less. */
+    --ui-tab-hover-darken: 2.5%;
     --dt-ring: var(--ui-stroke-primary);
     --dt-midground: var(--theme-midground);
     --dt-composer-ring: var(--ui-base);
@@ -446,6 +450,7 @@
     --backdrop-invert-mul: 0;
     /* Dark mode: a dark card needs a stronger black inset to show the recess. */
     --dt-input-inset: inset 0 1px 1px color-mix(in srgb, #000 38%, transparent);
+    --ui-tab-hover-darken: 6%;
     /* Dark needs a lighter resting border than light mode. */
     --dt-input-border: 4%;