fix(session): prevent input window from reopening in child sessions (#387)

Pressing <tab> or triggering focus_input in a child session could
re-show the input window that was intentionally hidden on session
switch. The root cause was that toggle_pane, ui.focus_input, and
input_window._show had no awareness of child session state.

Add child-session guards at three layers:
- input_window._show(): safety net at the lowest level, prevents the
  window from ever being recreated during a child session
- ui.focus_input(): early return so service-layer callers (agent_model,
  session_runtime) cannot accidentally reveal the input
- ui.toggle_pane(): early return so <tab> stays on the output pane

Author: hermandavid

lua/opencode/ui/input_window.lua

@@ -591,6 +591,11 @@ end
 
 ---Show the input window by recreating it
 function M._show()
+  -- Child sessions must never show the input window
+  if state.active_session and state.active_session.parentID then
+    return
+  end
+
   local windows = state.windows
   if not windows or not windows.input_buf or not windows.output_win then
     return

lua/opencode/ui/ui.lua

@@ -403,6 +403,10 @@ end
 
 ---@param opts? { restore_position?: boolean, start_insert?: boolean }
 function M.focus_input(opts)
+  if state.active_session and state.active_session.parentID then
+    return
+  end
+
   opts = opts or {}
   local windows = state.windows
   if not windows then
@@ -557,6 +561,9 @@ function M.toggle_pane()
   if state.windows and current_win == state.windows.input_win then
     output_window.focus_output(true)
   else
+    if state.active_session and state.active_session.parentID then
+      return
+    end
     input_window.focus_input()
   end
 end

tests/unit/input_window_spec.lua

@@ -548,4 +548,30 @@ describe('input_window', function()
       assert.is_false(input_window._hidden)
     end)
   end)
+
+  describe('child session input visibility', function()
+    after_each(function()
+      state.session.clear_active()
+    end)
+
+    it('_show() is a no-op when active session is a child session', function()
+      state.session.set_active({ id = 'child1', parentID = 'parent1' })
+      input_window._hidden = true
+
+      input_window._show()
+
+      assert.is_true(input_window._hidden)
+    end)
+
+    it('_show() proceeds when active session is a root session', function()
+      state.session.set_active({ id = 'root1' })
+      input_window._hidden = true
+
+      -- _show will early-return due to missing windows, but it should pass the guard
+      input_window._show()
+
+      -- _hidden remains true because windows are nil, but the parentID guard was passed
+      assert.is_true(input_window._hidden)
+    end)
+  end)
 end)

tests/unit/services_session_runtime_spec.lua

@@ -398,6 +398,39 @@ describe('opencode.services.session_runtime', function()
     end)
   end)
 
+  describe('child session UI guards', function()
+    local input_window = require('opencode.ui.input_window')
+
+    after_each(function()
+      state.session.clear_active()
+    end)
+
+    it('toggle_pane does not show input when in a child session', function()
+      state.session.set_active({ id = 'child1', parentID = 'parent1' })
+      stub(input_window, 'focus_input')
+
+      -- Simulate being in the output window (not input)
+      state.ui.set_windows({ input_win = -1, output_win = vim.api.nvim_get_current_win(), input_buf = 1, output_buf = 2 })
+
+      ui.toggle_pane()
+
+      assert.stub(input_window.focus_input).was_not_called()
+      input_window.focus_input:revert()
+    end)
+
+    it('focus_input is a no-op when in a child session', function()
+      state.session.set_active({ id = 'child1', parentID = 'parent1' })
+      stub(input_window, 'is_hidden').returns(true)
+      stub(input_window, '_show')
+
+      ui.focus_input()
+
+      assert.stub(input_window._show).was_not_called()
+      input_window.is_hidden:revert()
+      input_window._show:revert()
+    end)
+  end)
+
   describe('send_message', function()
     it('delegates message-sending coverage to services_messaging_spec', function()
       -- This spec focuses on session_runtime responsibilities.