screen.lua: expect_unchanged(), get_snapshot()

Factor `get_snapshot()` out of `print_snapshot()`, so that callers can
get a table (for use with `expect()`) instead of the string form.

Try to use this to fix indeterminism in `searchhl_spec.lua`.
  - Since the screen state is collected by `screen:expect_unchanged()`,
    we don't need a deterministic initial state (which would then be
    hardcoded into the test). This allows us to check "did anything
    change in the last N ms?" rather than "did anything change compared
    to a hardcoded screen-state?"
  - This may end up fruitless, because `expect_unchanged()` depends on
    timing to wait for an initial "current state".

test/functional/ui/screen.lua

@@ -447,6 +447,18 @@ screen:redraw_debug() to show all intermediate screen states.  ]])
   end, expected)
 end
 
+function Screen:expect_unchanged(waittime_ms, ignore_attrs, request_cb)
+  waittime_ms = waittime_ms and waittime_ms or 100
+  -- Collect the current screen state.
+  self:sleep(waittime_ms, request_cb)
+  local kwargs = self:get_snapshot(nil, ignore_attrs)
+  -- Wait for potential changes.
+  self:sleep(waittime_ms, request_cb)
+  kwargs.unchanged = true
+  -- Check that screen state did not change.
+  self:expect(kwargs)
+end
+
 function Screen:_wait(check, flags)
   local err, checked = false, false
   local success_seen = false
@@ -477,8 +489,8 @@ function Screen:_wait(check, flags)
     immediate_seen = true
   end
 
-  -- for an unchanged test, flags.timeout means the time during the state is
+  -- For an "unchanged" test, flags.timeout is the time during which the state
-  -- expected to be unchanged, so always wait this full time.
+  -- must not change, so always wait this full time.
   if (flags.unchanged or flags.intermediate) and flags.timeout ~= nil then
     minimal_timeout = timeout
   end
@@ -1214,7 +1226,9 @@ local remove_all_metatables = function(item, path)
   if path[#path] ~= inspect.METATABLE then return item end
 end
 
-function Screen:print_snapshot(attrs, ignore)
+-- Returns the current screen state in the form of a screen:expect()
+-- keyword-args map.
+function Screen:get_snapshot(attrs, ignore)
   attrs = attrs or self._default_attr_ids
   if ignore == nil then
     ignore = self._default_attr_ignore
@@ -1246,6 +1260,26 @@ function Screen:print_snapshot(attrs, ignore)
     end
   end
 
+  -- Build keyword-args for screen:expect().
+  local kwargs = {}
+  if attr_state.modified then
+    kwargs['attr_ids'] = {}
+    for i, a in pairs(attr_state.ids) do
+      kwargs['attr_ids'][i] = a
+    end
+  end
+  kwargs['grid'] = table.concat(lines, '\n')
+  for _, k in ipairs(ext_keys) do
+    if ext_state[k] ~= nil then
+      kwargs[k] = ext_state[k]
+    end
+  end
+
+  return kwargs, ext_state, attr_state, keys
+end
+
+function Screen:print_snapshot(attrs, ignore)
+  local kwargs, ext_state, attr_state, keys = self:get_snapshot(attrs, ignore)
   local attrstr = ""
   if attr_state.modified then
     local attrstrs = {}
@@ -1263,7 +1297,7 @@ function Screen:print_snapshot(attrs, ignore)
                .."{\n"..table.concat(attrstrs, "\n").."\n}")
   end
   print( "\nscreen:expect"..(keys and "{grid=" or "(").."[[")
-  print( table.concat(lines, '\n'))
+  print(kwargs.grid)
   io.stdout:write( "]]"..attrstr)
   for _, k in ipairs(ext_keys) do
     if ext_state[k] ~= nil then

test/functional/ui/searchhl_spec.lua

@@ -5,8 +5,7 @@ local command = helpers.command
 local feed_command = helpers.feed_command
 local eq = helpers.eq
 local eval = helpers.eval
-local iswin = helpers.iswin
+local nvim_dir = helpers.nvim_dir
-local sleep = helpers.sleep
 
 describe('search highlighting', function()
   local screen
@@ -148,11 +147,7 @@ describe('search highlighting', function()
   end)
 
   it('is preserved during :terminal activity', function()
-    if iswin() then
+    feed([[:terminal "]]..nvim_dir..[[/shell-test" REP 5000 foo<cr>]])
-      feed([[:terminal for /L \%I in (1,1,5000) do @(echo xxx & echo xxx & echo xxx)<cr>]])
-    else
-      feed([[:terminal for i in $(seq 1 5000); do printf 'xxx\nxxx\nxxx\n'; done<cr>]])
-    end
 
     feed(':file term<CR>')
     feed(':vnew<CR>')
@@ -162,23 +157,7 @@ describe('search highlighting', function()
       bar foo baz
     ]])
     feed('/foo')
-    sleep(50)  -- Allow some terminal activity.
+    screen:expect_unchanged()
-    -- NB: in earlier versions terminal output was redrawn during cmdline mode.
-    -- For now just assert that the screens remain unchanged.
-    screen:expect([[
-        {3:foo} bar baz       {3:│}                   |
-        bar baz {2:foo}       {3:│}                   |
-        bar {2:foo} baz       {3:│}                   |
-                          {3:│}                   |
-      {1:~                   }{3:│}                   |
-      {5:[No Name] [+]        }{3:term               }|
-      /foo^                                    |
-    ]], { [1] = {bold = true, foreground = Screen.colors.Blue1},
-          [2] = {background = Screen.colors.Yellow},
-          [3] = {reverse = true},
-          [4] = {foreground = Screen.colors.Red},
-          [5] = {bold = true, reverse = true},
-    })
   end)
 
   it('works with incsearch', function()