fix: hide input window when viewing child sessions

Child sessions are read-only, so the input window should be hidden
when switching to one, and restored when switching back to a regular
session. This prevents the user from attempting to type in a session
that cannot accept messages.

- Hide input and focus output when switching to a child session
- Show input and focus it when switching back to a non-child session
- Skip redundant hide/show when window is already in the right state

Author: hermandavid

lua/opencode/services/session_runtime.lua

@@ -49,7 +49,17 @@ M.switch_session = Promise.async(function(session_id)
 
   state.session.set_active(selected_session)
   if state.ui.is_visible() then
-    ui.focus_input()
+    if selected_session and selected_session.parentID then
+      if not input_window.is_hidden() then
+        input_window._hide()
+      end
+      ui.focus_output()
+    else
+      if input_window.is_hidden() then
+        input_window._show()
+      end
+      ui.focus_input()
+    end
   else
     M.open()
   end

tests/unit/services_session_runtime_spec.lua

@@ -333,6 +333,71 @@ describe('opencode.services.session_runtime', function()
     end)
   end)
 
+  describe('switch_session', function()
+    local input_window = require('opencode.ui.input_window')
+
+    it('hides input window when switching to a child session', function()
+      state.ui.set_windows({ mock = 'windows', input_buf = 1, output_buf = 2, input_win = 3, output_win = 4 })
+      local orig_is_visible = state.ui.is_visible
+      state.ui.is_visible = function() return true end
+      stub(input_window, 'is_hidden').returns(false)
+      stub(input_window, '_hide')
+
+      session.get_by_id:revert()
+      stub(session, 'get_by_id').invokes(function(id)
+        return Promise.new():resolve({ id = id, title = id, modified = os.time(), parentID = 'parent1' })
+      end)
+
+      session_runtime.switch_session('child1'):wait()
+
+      assert.stub(input_window._hide).was_called()
+      assert.stub(ui.focus_output).was_called()
+
+      input_window.is_hidden:revert()
+      input_window._hide:revert()
+      state.ui.is_visible = orig_is_visible
+    end)
+
+    it('shows input window when switching to a non-child session', function()
+      state.ui.set_windows({ mock = 'windows', input_buf = 1, output_buf = 2, input_win = 3, output_win = 4 })
+      local orig_is_visible = state.ui.is_visible
+      state.ui.is_visible = function() return true end
+      stub(input_window, 'is_hidden').returns(true)
+      stub(input_window, '_show')
+
+      session_runtime.switch_session('root1'):wait()
+
+      assert.stub(input_window._show).was_called()
+      assert.stub(ui.focus_input).was_called()
+
+      input_window.is_hidden:revert()
+      input_window._show:revert()
+      state.ui.is_visible = orig_is_visible
+    end)
+
+    it('does not hide input when already hidden on child session switch', function()
+      state.ui.set_windows({ mock = 'windows', input_buf = 1, output_buf = 2, input_win = 3, output_win = 4 })
+      local orig_is_visible = state.ui.is_visible
+      state.ui.is_visible = function() return true end
+      stub(input_window, 'is_hidden').returns(true)
+      stub(input_window, '_hide')
+
+      session.get_by_id:revert()
+      stub(session, 'get_by_id').invokes(function(id)
+        return Promise.new():resolve({ id = id, title = id, modified = os.time(), parentID = 'parent1' })
+      end)
+
+      session_runtime.switch_session('child1'):wait()
+
+      assert.stub(input_window._hide).was_not_called()
+      assert.stub(ui.focus_output).was_called()
+
+      input_window.is_hidden:revert()
+      input_window._hide:revert()
+      state.ui.is_visible = orig_is_visible
+    end)
+  end)
+
   describe('send_message', function()
     it('delegates message-sending coverage to services_messaging_spec', function()
       -- This spec focuses on session_runtime responsibilities.