fix(desktop): keep the chat scrollbar off the right-sidebar sash

The vertical sash grab band sat 1px into the leading pane and stole
the Windows overlay-scrollbar hit target. Keep the 8px band entirely
on the trailing side of the seam.

apps/desktop/src/components/pane-shell/tree/renderer/main-placement-floor.test.tsx

@@ -96,3 +96,23 @@ describe('the main placement floor in the renderer', () => {
     expect(zoneWrapper('grp-tools')?.style.display).toBe('none')
   })
 })
+
+describe('vertical sash vs leading-pane scrollbar (#99867)', () => {
+  it('keeps the grab band entirely in the trailing pane', () => {
+    const tree = split('row', [
+      group(['sessions'], { id: 'grp-sessions' }),
+      group(['workspace'], { id: 'grp-main' })
+    ])
+
+    $layoutTree.set(tree)
+    render(<TreeSplit node={tree} root rootRow />)
+
+    const sashes = document.querySelectorAll('[data-sash-overlap="trailing"]')
+
+    expect(sashes.length).toBeGreaterThan(0)
+    for (const sash of sashes) {
+      expect(sash.className).toContain('w-[8px]')
+      expect(sash.className).not.toContain('-translate-x-[1px]')
+    }
+  })
+})

apps/desktop/src/components/pane-shell/tree/renderer/tree-split.tsx

@@ -812,14 +812,15 @@ function Sash({
     <div
       className={cn(
         'group absolute z-20 [-webkit-app-region:no-drag]',
-        // Asymmetric grab band: only 1px reaches into the leading pane so its
-        // edge-hugging 8px scrollbar stays clickable (the old centered 9px band
-        // swallowed it entirely — the pointer got col-resize instead of the
-        // thumb). The trailing side keeps a generous 7px reach; total grab
-        // width stays ~8px so the sash is no harder to hit.
-        horizontal ? 'inset-y-0 left-0 w-[8px] -translate-x-[1px]' : 'inset-x-0 top-0 h-[8px] -translate-y-[1px]',
+        // Grab band lives entirely in the trailing pane. A 1px overlap into
+        // the leading pane (the previous asymmetric band) still stole the
+        // Windows overlay-scrollbar hit target on the chat — only ~3px of
+        // thumb remained clickable (#99867). The sash stays 8px wide, all
+        // on the sidebar/tool side of the seam.
+        horizontal ? 'inset-y-0 left-0 w-[8px]' : 'inset-x-0 top-0 h-[8px]',
         disabled ? 'pointer-events-none' : horizontal ? 'cursor-col-resize' : 'cursor-row-resize'
       )}
+      data-sash-overlap="trailing"
       onDoubleClick={disabled ? undefined : onDoubleClick}
       onPointerDown={disabled ? undefined : onPointerDown}
       role="separator"