fix(desktop): open a preview without dragging the file tree open

The preview pane shares a collapsible column with the file tree, and
`revealTreePane` un-collapses a column through that column's bound store —
which on the right is `$fileBrowserOpen`, the tree's own ⌘J toggle. So
every preview open literally called `setFileBrowserOpen(true)` and the tree
came with it.

`revealPreview` now un-collapses the column directly and leaves the toggle
alone. The tree pane's visibility binding gains `$fileBrowserOpen` to match,
since its presence was tracking only the column's collapse — without that it
would still render the moment anything opened the column.

apps/desktop/src/app/contrib/controller.tsx

@@ -27,6 +27,8 @@ import {
   revealTreePane,
   setPaneCollapsed,
   setTreePaneHidden,
+  setTreeSideCollapsed,
+  treeSideOfPane,
   watchContributedPanes
 } from '@/components/pane-shell/tree/store'
 import { SidebarProvider } from '@/components/ui/sidebar'
@@ -533,7 +535,13 @@ bindTreeSideVisibility('right', $fileBrowserOpen, setFileBrowserOpen)
 // rode the rail's row and vanished with it), its zone stands on its own.
 const $hasWorkspace = computed($currentCwd, cwd => Boolean(cwd.trim()))
 
-bindPaneVisibility('files', $hasWorkspace)
+// The tree pane's own presence tracks ⌘J directly, not just the column's
+// collapse — otherwise revealing a preview (which opens that shared column)
+// would drag the tree along with it. See revealPreview.
+bindPaneVisibility(
+  'files',
+  computed([$hasWorkspace, $fileBrowserOpen], (workspace, open) => workspace && open)
+)
 // ⌘G — the review sidebar appears/disappears (and comes to the front).
 bindPaneVisibility(
   'review',
@@ -596,6 +604,18 @@ registerPaneCloser('files', () =>
 // the side, unhide, front — a NEW target while already visible still fronts.
 const revealPreview = () => {
   dockPaneBeside('preview', 'files')
+
+  // The preview shares a collapsible column with the file tree, and
+  // revealTreePane un-collapses a column through its bound store — here ⌘J /
+  // $fileBrowserOpen, which IS the tree's toggle. Going through it would open
+  // the tree every time a preview opened. Un-collapse the column directly and
+  // leave the toggle alone, so a preview can appear on its own.
+  const side = treeSideOfPane('preview')
+
+  if (side) {
+    setTreeSideCollapsed(side, false)
+  }
+
   revealTreePane('preview')
 }
 

apps/desktop/src/components/pane-shell/tree/reactive-unhide.test.ts

@@ -127,6 +127,38 @@ describe('reactive pane unhide', () => {
     expect(tree.$collapsedTreeSides.get().has('right')).toBe(false)
   })
 
+  // Opening a preview used to drag the file tree open with it: the preview
+  // shares ⌘J's column, and `revealTreePane` un-collapses a column through its
+  // bound store — which for the right side IS the file-browser toggle. The
+  // reveal now un-collapses the column directly and leaves the toggle alone.
+  it('revealing a preview opens its column without flipping the file-tree toggle', async () => {
+    const { tree, layout } = await setupWithFiles()
+    const { registry } = await import('@/contrib/registry')
+
+    registry.register({
+      id: 'preview',
+      area: 'panes',
+      data: { placement: 'right' },
+      render: () => null,
+      title: 'preview'
+    })
+
+    layout.setFileBrowserOpen(false)
+    expect(tree.$collapsedTreeSides.get().has('right')).toBe(true)
+
+    // The revealPreview sequence from controller.tsx.
+    const side = tree.treeSideOfPane('files')
+
+    expect(side).toBe('right')
+    tree.setTreeSideCollapsed(side!, false)
+    tree.revealTreePane('preview')
+
+    // The column is showing…
+    expect(tree.$collapsedTreeSides.get().has('right')).toBe(false)
+    // …but the tree's own toggle never moved, so the tree stays closed.
+    expect(layout.$fileBrowserOpen.get()).toBe(false)
+  })
+
   it('reactive unhide does not invoke the right side opener directly', async () => {
     const { tree, layout } = await setupWithFiles()