fix(tui): initialize clear attrs with current terminal background

Problem:  Invalidated regions that are flushed during startup are
          cleared with unitialized "clear_attrs", which is perceived as
          flickering.
Solution: Initialize "clear_attrs" with current terminal background color.

src/nvim/tui/tui.c

@@ -23,6 +23,7 @@
 #include "nvim/globals.h"
 #include "nvim/grid.h"
 #include "nvim/grid_defs.h"
+#include "nvim/highlight.h"
 #include "nvim/highlight_defs.h"
 #include "nvim/log.h"
 #include "nvim/macros_defs.h"
@@ -165,6 +166,15 @@ void tui_start(TUIData **tui_p, int *width, int *height, char **term, bool *rgb)
   tui->seen_error_exit = 0;
   tui->loop = &main_loop;
   tui->url = -1;
+  // Because setting the default colors is delayed until after startup to avoid
+  // flickering with the default colorscheme background, any flush that happens
+  // during startup in turn would result in clearing invalidated regions with
+  // uninitialized attrs(black). Instead initialize clear_attrs with current
+  // terminal background so that it is at least not perceived as flickering, even
+  // though it may be different from the colorscheme that is set during startup.
+  tui->clear_attrs.rgb_bg_color = normal_bg;
+  tui->clear_attrs.cterm_bg_color = (int16_t)cterm_normal_bg_color;
+
   kv_init(tui->invalid_regions);
   kv_init(tui->urlbuf);
   signal_watcher_init(tui->loop, &tui->winch_handle, tui);

test/functional/terminal/tui_spec.lua

@@ -2003,6 +2003,39 @@ describe('TUI', function()
     ]])
   end)
 
+  it('invalidated regions are cleared with terminal background attr', function()
+    local screen = Screen.new(50, 10)
+    screen:set_default_attr_ids({ [1] = { foreground = Screen.colors.Black } })
+    screen:attach()
+    fn.termopen({
+      nvim_prog,
+      '--clean',
+      '--cmd',
+      'set termguicolors',
+      '--cmd',
+      'sleep 10',
+    }, {
+      env = {
+        VIMRUNTIME = os.getenv('VIMRUNTIME'),
+      },
+    })
+    screen:expect({
+      grid = [[
+        {1:^                                                  }|
+        {1:                                                  }|*8
+                                                          |
+      ]],
+    })
+    screen:try_resize(51, 11)
+    screen:expect({
+      grid = [[
+        {1:^                                                   }|
+        {1:                                                   }|*9
+                                                           |
+      ]],
+    })
+  end)
+
   it('argv[0] can be overridden #23953', function()
     if not exec_lua('return pcall(require, "ffi")') then
       pending('missing LuaJIT FFI')