term: after <C-\>, resume normal input loop

Pressing <C-\> and then a mouse click will insert the click into the
terminal as if a keyboard button had been pressed.

Keep track of whether the last input was <C-\> and only call
terminal_send_key() if the next input is a key press.

src/nvim/terminal.c

@@ -376,6 +376,8 @@ void terminal_enter(bool process_deferred)
   int c;
   bool close = false;
 
+  bool got_bs = false;  // True if the last input was <C-\>
+
   while (term->buf == curbuf) {
     if (process_deferred) {
       event_enable_deferred();
@@ -388,14 +390,6 @@ void terminal_enter(bool process_deferred)
     }
 
     switch (c) {
-      case Ctrl_BSL:
-        c = safe_vgetc();
-        if (c == Ctrl_N) {
-          goto end;
-        }
-        terminal_send_key(term, c);
-        break;
-
       case K_LEFTMOUSE:
       case K_LEFTDRAG:
       case K_LEFTRELEASE:
@@ -416,12 +410,22 @@ void terminal_enter(bool process_deferred)
         event_process();
         break;
 
+      case Ctrl_N:
+        if (got_bs) {
+          goto end;
+        }
+
       default:
+        if (c == Ctrl_BSL && !got_bs) {
+          got_bs = true;
+          break;
+        }
         if (term->closed) {
           close = true;
           goto end;
         }
 
+        got_bs = false;
         terminal_send_key(term, c);
     }
   }

test/functional/terminal/buffer_spec.lua

@@ -170,15 +170,18 @@ describe('terminal buffer', function()
     source([[
     function! SplitWindow()
       new
+      call feedkeys("iabc\<Esc>")
     endfunction
 
     startinsert
     call jobstart(['sh', '-c', 'exit'], {'on_exit': function("SplitWindow")})
+    call feedkeys("\<C-\>", 't')  " vim will expect <C-n>, but be exited out of
+                                  " the terminal before it can be entered.
     ]])
 
     -- We should be in a new buffer now.
     screen:expect([[
-      ^                                                  |
+      ab^c                                               |
       ~                                                 |
       ==========                                        |
       rows: 2, cols: 50                                 |
@@ -188,7 +191,7 @@ describe('terminal buffer', function()
     ]])
 
     neq(tbuf, eval('bufnr("%")'))
-    execute('quit')  -- Should exit the new window, not the terminal.
+    execute('quit!')  -- Should exit the new window, not the terminal.
     eq(tbuf, eval('bufnr("%")'))
 
     execute('set laststatus=1')  -- Restore laststatus to the default.

test/functional/terminal/mouse_spec.lua

@@ -50,6 +50,20 @@ describe('terminal mouse', function()
       ]])
     end)
 
+    it('will exit focus after <C-\\>, then scrolled', function()
+      feed('<C-\\>')
+      feed('<MouseDown><0,0>')
+      screen:expect([[
+        line23                                            |
+        line24                                            |
+        line25                                            |
+        line26                                            |
+        line27                                            |
+        ^line28                                            |
+                                                          |
+      ]])
+    end)
+
     describe('with mouse events enabled by the program', function()
       before_each(function()
         thelpers.enable_mouse()