fix: preserve terminal window size across session operations

Add dedicated window_manager module that owns the single terminal window.
This separates window lifecycle from buffer lifecycle, fixing issues where:
- Creating new sessions would reset window to default size
- Switching between tabs could cause window duplication
- Closing tabs could leave windows in wrong positions

Changes:
- Add window_manager.lua: singleton that manages THE terminal window
- Refactor snacks.lua: create buffers only, delegate window to manager
- Refactor native.lua: simplified buffer-only management
- Update terminal.lua: initialize window_manager, improve tab navigation
- New tab now selects the created session
- Closing tab selects previous tab (or next if first)

Author: Snir Turgeman

ARCHITECTURE.md

@@ -129,22 +129,36 @@ vim.api.nvim_create_autocmd("CursorMoved", {
 -- Preserves selection context when switching to terminal
 ```
 
-### 6. Terminal Integration (`terminal.lua`)
+### 6. Terminal Integration (`terminal/`)
 
-Flexible terminal management with provider pattern:
+Flexible terminal management with provider pattern and centralized window management:
 
 ```lua
--- Snacks.nvim provider (preferred)
-if has_snacks then
-  Snacks.terminal.open(cmd, {
-    win = { position = "right", width = 0.3 }
-  })
-else
-  -- Native fallback
-  vim.cmd("vsplit | terminal " .. cmd)
-end
+-- Window manager singleton owns THE terminal window
+-- Providers only create buffers, window_manager displays them
+local window_manager = require("claudecode.terminal.window_manager")
+
+-- Display a buffer in the managed window (preserves user resizing)
+window_manager.display_buffer(bufnr, focus)
+
+-- Snacks.nvim provider creates buffer, delegates window to manager
+local term = Snacks.terminal.open(cmd, opts)
+vim.api.nvim_win_close(term.win, false)  -- Close snacks' window
+window_manager.display_buffer(term.buf, true)  -- Use our window
+
+-- Native provider creates buffer without window
+local bufnr = vim.api.nvim_create_buf(false, true)
+vim.fn.termopen(cmd, { env = env })
+window_manager.display_buffer(bufnr, true)
 ```
 
+Key features:
+
+- **Single window**: All sessions share one terminal window
+- **Buffer switching**: `nvim_win_set_buf()` preserves window size
+- **Session management**: Multiple Claude sessions with tab-like switching
+- **Window preservation**: User resizing persists across session switches
+
 ## Key Implementation Patterns
 
 ### Thread Safety
@@ -199,7 +213,15 @@ lua/claudecode/
 ├── tools/init.lua        # MCP tool registry
 ├── diff.lua              # Native diff support
 ├── selection.lua         # Selection tracking
-├── terminal.lua          # Terminal management
+├── session.lua           # Multi-session state management
+├── terminal.lua          # Terminal orchestration
+├── terminal/             # Terminal providers
+│   ├── window_manager.lua # Singleton window management
+│   ├── snacks.lua        # Snacks.nvim provider
+│   ├── native.lua        # Native Neovim terminal
+│   ├── external.lua      # External terminal apps
+│   ├── tabbar.lua        # Session tab bar UI
+│   └── osc_handler.lua   # Terminal title detection
 └── lockfile.lua          # Discovery files
 ```
 

lua/claudecode/terminal.lua

@@ -777,6 +777,13 @@ function M.setup(user_term_config, p_terminal_cmd, p_env)
     end
   end
 
+  -- Setup window manager with config
+  local window_manager = require("claudecode.terminal.window_manager")
+  window_manager.setup({
+    split_side = defaults.split_side,
+    split_width_percentage = defaults.split_width_percentage,
+  })
+
   -- Setup providers with config
   get_provider().setup(defaults)
 
@@ -831,6 +838,7 @@ end
 ---Closes the managed Claude terminal if it's open and valid.
 function M.close()
   detach_tabbar()
+  -- Call provider's close for backwards compatibility
   get_provider().close()
 end
 
@@ -998,14 +1006,17 @@ function M.open_new_session(opts_override, cmd_args)
   local effective_config = build_config(opts_override)
   local cmd_string, claude_env_table = get_claude_command_and_env(cmd_args)
 
+  -- Make the new session active immediately
+  session_manager.set_active_session(session_id)
+
   local provider = get_provider()
 
   -- For multi-session, we need to pass session_id to providers
   if provider.open_session then
-    provider.open_session(session_id, cmd_string, claude_env_table, effective_config)
+    provider.open_session(session_id, cmd_string, claude_env_table, effective_config, true) -- true = focus
   else
     -- Fallback: use regular open (single terminal mode)
-    provider.open(cmd_string, claude_env_table, effective_config)
+    provider.open(cmd_string, claude_env_table, effective_config, true) -- true = focus
   end
 
   return session_id
@@ -1020,41 +1031,94 @@ function M.close_session(session_id)
   end
 
   local provider = get_provider()
+  local effective_config = build_config(nil)
+
+  -- Check if there are other sessions to switch to
+  local session_count = session_manager.get_session_count()
+
+  if session_count > 1 then
+    -- There are other sessions - keep the window and switch to another session
+    -- Figure out which session to switch to: prefer previous tab, fallback to next
+    local sessions = session_manager.list_sessions()
+    local new_active_id = nil
+    local current_index = nil
+
+    -- Find the index of the session being closed
+    for i, s in ipairs(sessions) do
+      if s.id == session_id then
+        current_index = i
+        break
+      end
+    end
 
-  -- Detach tabbar before closing the terminal window
-  detach_tabbar()
+    if current_index then
+      -- Prefer previous tab (index - 1), fallback to next tab (index + 1)
+      if current_index > 1 then
+        new_active_id = sessions[current_index - 1].id
+      elseif current_index < #sessions then
+        new_active_id = sessions[current_index + 1].id
+      end
+    end
 
-  if provider.close_session then
-    provider.close_session(session_id)
-  else
-    -- Fallback: use regular close
-    provider.close()
-  end
+    -- Fallback: just pick any other session
+    if not new_active_id then
+      for _, s in ipairs(sessions) do
+        if s.id ~= session_id then
+          new_active_id = s.id
+          break
+        end
+      end
+    end
+
+    if new_active_id and provider.close_session_keep_window then
+      -- Use close_session_keep_window to keep window open and switch buffer
+      -- This function handles cleanup of the old session internally
+      provider.close_session_keep_window(session_id, new_active_id, effective_config)
+      session_manager.destroy_session(session_id)
+      session_manager.set_active_session(new_active_id)
+    else
+      -- Fallback: close and reopen
+      session_manager.destroy_session(session_id)
+      new_active_id = session_manager.get_active_session_id()
 
-  session_manager.destroy_session(session_id)
+      if provider.close_session then
+        provider.close_session(session_id)
+      else
+        provider.close()
+      end
 
-  -- If there are remaining sessions, switch to the new active session
-  local new_active_id = session_manager.get_active_session_id()
-  if new_active_id then
-    local effective_config = build_config(nil)
-    if provider.focus_session then
-      provider.focus_session(new_active_id, effective_config)
+      if new_active_id and provider.focus_session then
+        provider.focus_session(new_active_id, effective_config)
+      end
     end
 
     -- Re-attach tabbar to the new session's terminal
-    local new_bufnr
-    if provider.get_session_bufnr then
-      new_bufnr = provider.get_session_bufnr(new_active_id)
-    else
-      new_bufnr = provider.get_active_bufnr()
-    end
+    if new_active_id then
+      local new_bufnr
+      if provider.get_session_bufnr then
+        new_bufnr = provider.get_session_bufnr(new_active_id)
+      else
+        new_bufnr = provider.get_active_bufnr()
+      end
 
-    if new_bufnr and vim.fn.getbufinfo then
-      local ok, bufinfo = pcall(vim.fn.getbufinfo, new_bufnr)
-      if ok and bufinfo and #bufinfo > 0 and #bufinfo[1].windows > 0 then
-        attach_tabbar(bufinfo[1].windows[1], new_bufnr)
+      if new_bufnr and vim.fn.getbufinfo then
+        local ok, bufinfo = pcall(vim.fn.getbufinfo, new_bufnr)
+        if ok and bufinfo and #bufinfo > 0 and #bufinfo[1].windows > 0 then
+          attach_tabbar(bufinfo[1].windows[1], new_bufnr)
+        end
       end
     end
+  else
+    -- This is the last session - close everything
+    detach_tabbar()
+
+    if provider.close_session then
+      provider.close_session(session_id)
+    else
+      provider.close()
+    end
+
+    session_manager.destroy_session(session_id)
   end
 end