fix(ui): flush ext_cmdline events before doing cmdpreview #27950

Problem:  Unable to update the screen for external cmdline during cmdpreview.
Solution: Flush the cmdline UI before cmdpreview state.

src/nvim/ex_getln.c

@@ -2532,6 +2532,10 @@ static bool cmdpreview_may_show(CommandLineState *s)
     goto end;
   }
 
+  // Flush now: external cmdline may itself wish to update the screen which is
+  // currently disallowed during cmdpreview(no longer needed in case that changes).
+  cmdline_ui_flush();
+
   // Swap invalid command range if needed
   if ((ea.argt & EX_RANGE) && ea.line1 > ea.line2) {
     linenr_T lnum = ea.line1;

test/functional/lua/ui_event_spec.lua

@@ -37,6 +37,9 @@ describe('vim.ui_attach', function()
       [2] = { bold = true },
       [3] = { background = Screen.colors.Grey },
       [4] = { background = Screen.colors.LightMagenta },
+      [5] = { reverse = true },
+      [6] = { reverse = true, bold = true },
+      [7] = { background = Screen.colors.Yellow1 },
     })
     screen:attach()
   end)
@@ -188,6 +191,56 @@ describe('vim.ui_attach', function()
     feed('version<CR><CR>v<Esc>')
     n.assert_alive()
   end)
+
+  it("preserved 'incsearch/command' screen state after :redraw from ext_cmdline", function()
+    exec_lua([[
+      vim.cmd.norm('ifoobar')
+      vim.cmd('1split cmdline')
+      local buf = vim.api.nvim_get_current_buf()
+      vim.cmd.wincmd('p')
+      vim.ui_attach(ns, { ext_cmdline = true }, function(event, ...)
+        if event == 'cmdline_show' then
+          local content = select(1, ...)
+          vim.api.nvim_buf_set_lines(buf, -2, -1, false, {content[1][2]})
+          vim.cmd('redraw')
+        end
+        return true
+      end)
+    ]])
+    -- Updates a cmdline window
+    feed(':cmdline')
+    screen:expect({
+      grid = [[
+        cmdline                                 |
+        {5:cmdline [+]                             }|
+        fooba^r                                  |
+        {6:[No Name] [+]                           }|
+                                                |
+      ]],
+    })
+    -- Does not clear 'incsearch' highlighting
+    feed('<Esc>/foo')
+    screen:expect({
+      grid = [[
+        foo                                     |
+        {5:cmdline [+]                             }|
+        {5:foo}ba^r                                  |
+        {6:[No Name] [+]                           }|
+                                                |
+      ]],
+    })
+    -- Shows new cmdline state during 'inccommand'
+    feed('<Esc>:%s/bar/baz')
+    screen:expect({
+      grid = [[
+        %s/bar/baz                              |
+        {5:cmdline [+]                             }|
+        foo{7:ba^z}                                  |
+        {6:[No Name] [+]                           }|
+                                                |
+      ]],
+    })
+  end)
 end)
 
 describe('vim.ui_attach', function()