fix(float): tail truncate the long title

Problem: now we truncate the title from start,This is somewhat inconsistent with the way vim works. Usually we use < to truncate and keep more trailing characters.

Solution: keep more trailing characters.

src/nvim/drawscreen.c

@@ -92,6 +92,7 @@
 #include "nvim/match.h"
 #include "nvim/mbyte.h"
 #include "nvim/memline.h"
+#include "nvim/memory.h"
 #include "nvim/message.h"
 #include "nvim/move.h"
 #include "nvim/normal.h"
@@ -717,18 +718,49 @@ void end_search_hl(void)
 
 static void win_redr_bordertext(win_T *wp, VirtText vt, int col)
 {
+  int used_col = col;
+  int width = wp->w_config.width;
+  bool truncate = false;
+  char *new_text = NULL;
+
   for (size_t i = 0; i < kv_size(vt);) {
     int attr = 0;
     char *text = next_virt_text_chunk(vt, &i, &attr);
     if (text == NULL) {
       break;
     }
     attr = hl_apply_winblend(wp, attr);
-    col += grid_line_puts(col, text, -1, attr);
+    int cell = (int)mb_string2cells(text);
+    int shift = used_col + cell - width;
+
+    if (shift > 1) {
+      truncate = true;
+      if (used_col > col) {
+        int move_start = col + shift - 1;
+        int start_pos = col;
+        for (int j = move_start; j < used_col; j++) {
+          grid_line_put_schar(start_pos, linebuf_char[j], linebuf_attr[j]);
+          start_pos++;
+        }
+        used_col -= (shift - 1);
+      } else {
+        new_text = xmemdupz(text + shift - 1, strlen(text) - (size_t)shift + 1);
+      }
+    }
+
+    used_col += grid_line_puts(used_col, new_text ? new_text : text, -1, attr);
+    if (new_text) {
+      xfree(new_text);
+      new_text = NULL;
+    }
+  }
+
+  if (truncate) {
+    grid_line_put_schar(col, schar_from_ascii('<'), linebuf_attr[col]);
   }
 }
 
-int win_get_bordertext_col(int total_col, int text_width, AlignTextPos align)
+static int win_get_bordertext_col(int total_col, int text_width, AlignTextPos align)
 {
   switch (align) {
   case kAlignLeft:

test/functional/ui/float_spec.lua

@@ -1935,15 +1935,26 @@ describe('float window', function()
       eq('center', footer_pos)
     end)
 
-    it('center aligned title longer than window width #25746', function()
+    it('truncates title longer than window width #23602', function()
       local buf = api.nvim_create_buf(false, false)
       api.nvim_buf_set_lines(buf, 0, -1, true, {' halloj! ',
                                              ' BORDAA  '})
-      local win = api.nvim_open_win(buf, false, {
+      local config = {
         relative='editor', width=9, height=2, row=2, col=5, border="double",
         title = "abcdefghijklmnopqrstuvwxyz",title_pos = "center",
-      })
+      }
+
+      local expected = [[
+      ^                                        |
+      {0:~                                       }|
+      {0:~    }{5:╔}{11:<stuvwxyz}{5:╗}{0:                        }|
+      {0:~    }{5:║}{1: halloj! }{5:║}{0:                        }|
+      {0:~    }{5:║}{1: BORDAA  }{5:║}{0:                        }|
+      {0:~    }{5:╚═════════╝}{0:                        }|
+                                              |
+      ]]
 
+      local win = api.nvim_open_win(buf, false, config)
       if multigrid then
         screen:expect{grid=[[
         ## grid 1
@@ -1955,7 +1966,7 @@ describe('float window', function()
         ## grid 3
                                                   |
         ## grid 4
-          {5:╔}{11:abcdefghi}{5:╗}|
+          {5:╔}{11:<stuvwxyz}{5:╗}|
           {5:║}{1: halloj! }{5:║}|
           {5:║}{1: BORDAA  }{5:║}|
           {5:╚═════════╝}|
@@ -1966,17 +1977,38 @@ describe('float window', function()
           [4] = {win = 1001, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 2, sum_scroll_delta = 0};
         }}
       else
-        screen:expect{grid=[[
-          ^                                        |
-          {0:~                                       }|
-          {0:~    }{5:╔}{11:abcdefghi}{5:╗}{0:                        }|
-          {0:~    }{5:║}{1: halloj! }{5:║}{0:                        }|
-          {0:~    }{5:║}{1: BORDAA  }{5:║}{0:                        }|
-          {0:~    }{5:╚═════════╝}{0:                        }|
-                                                  |
-        ]]}
+        screen:expect{grid= expected}
       end
+      api.nvim_win_close(win, false)
+      assert_alive()
+
+      config.title = { {'abcd', 'FloatTitle'}, {'stuvw', 'FloatTitle'}, {'xyz', 'FloatTitle'}}
+      win = api.nvim_open_win(buf, false, config)
 
+      if multigrid then
+        screen:expect({grid = [[
+          ## grid 1
+            [2:----------------------------------------]|*6
+            [3:----------------------------------------]|
+          ## grid 2
+            ^                                        |
+            {0:~                                       }|*5
+          ## grid 3
+                                                    |
+          ## grid 5
+            {5:╔}{11:<stuvwxyz}{5:╗}|
+            {5:║}{1: halloj! }{5:║}|
+            {5:║}{1: BORDAA  }{5:║}|
+            {5:╚═════════╝}|
+          ]], float_pos={
+          [5] = {1002, "NW", 1, 2, 5, true, 50};
+        }, win_viewport={
+          [2] = {win = 1000, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 1, sum_scroll_delta = 0};
+          [5] = {win = 1002, topline = 0, botline = 2, curline = 0, curcol = 0, linecount = 2, sum_scroll_delta = 0};
+        }})
+      else
+        screen:expect{grid= expected}
+      end
       api.nvim_win_close(win, false)
       assert_alive()
     end)