fix(cmdline): ext_cmdline block events for conditionals

Problem:  No block events emitted with ext_cmdline for :if, :while, :try etc.
Solution: Emit cmdline block events; store the indent level of the
          previous cmdline and whether a block event was emitted.

src/nvim/ex_docmd.c

@@ -406,6 +406,7 @@ int do_cmdline(char *cmdline, LineGetter fgetline, void *cookie, int flags)
   bool msg_didout_before_start = false;
   int count = 0;                        // line number count
   bool did_inc = false;                 // incremented RedrawingDisabled
+  int block_indent = -1;                // indent for ext_cmdline block event
   int retval = OK;
   cstack_T cstack = {                   // conditional stack
     .cs_idx = -1,
@@ -573,16 +574,18 @@ int do_cmdline(char *cmdline, LineGetter fgetline, void *cookie, int flags)
 
     // 2. If no line given, get an allocated line with fgetline().
     if (next_cmdline == NULL) {
-      // Need to set msg_didout for the first line after an ":if",
-      // otherwise the ":if" will be overwritten.
-      if (count == 1 && getline_equal(fgetline, cookie, getexline)) {
-        msg_didout = true;
+      int indent = cstack.cs_idx < 0 ? 0 : (cstack.cs_idx + 1) * 2;
+      if (count >= 1 && getline_equal(fgetline, cookie, getexline)) {
+        if (ui_has(kUICmdline)) {
+          ui_ext_cmdline_block_append((size_t)MAX(0, block_indent), last_cmdline);
+          block_indent = indent;
+        } else if (count == 1) {
+          // Need to set msg_didout for the first line after an ":if",
+          // otherwise the ":if" will be overwritten.
+          msg_didout = true;
+        }
       }
-      if (fgetline == NULL
-          || (next_cmdline = fgetline(':', cookie,
-                                      cstack.cs_idx <
-                                      0 ? 0 : (cstack.cs_idx + 1) * 2,
-                                      true)) == NULL) {
+      if (fgetline == NULL || (next_cmdline = fgetline(':', cookie, indent, true)) == NULL) {
         // Don't call wait_return() for aborted command line.  The NULL
         // returned for the end of a sourced file or executed function
         // doesn't do this.
@@ -939,6 +942,10 @@ int do_cmdline(char *cmdline, LineGetter fgetline, void *cookie, int flags)
     }
   }
 
+  if (block_indent >= 0) {
+    ui_ext_cmdline_block_leave();
+  }
+
   did_endif = false;    // in case do_cmdline used recursively
 
   do_cmdline_end();

test/functional/ui/cmdline_spec.lua

@@ -865,6 +865,59 @@ local function test_cmdline(linegrid)
       },
     })
   end)
+
+  it('works with conditionals', function()
+    local s1 = [[
+      ^                         |
+      {1:~                        }|*3
+                               |
+    ]]
+    screen:expect(s1)
+    feed(':if 1<CR>')
+    screen:expect({
+      grid = s1,
+      cmdline = {
+        {
+          content = { { '' } },
+          firstc = ':',
+          indent = 2,
+          pos = 0,
+        },
+      },
+      cmdline_block = { { { 'if 1' } } },
+    })
+    feed(':let x = 1<CR>')
+    screen:expect({
+      grid = s1,
+      cmdline = {
+        {
+          content = { { '' } },
+          firstc = ':',
+          indent = 2,
+          pos = 0,
+        },
+      },
+      cmdline_block = { { { 'if 1' } }, { { '  :let x = 1' } } },
+    })
+    feed(':endif')
+    screen:expect({
+      grid = s1,
+      cmdline = {
+        {
+          content = { { ':endif' } },
+          firstc = ':',
+          indent = 2,
+          pos = 6,
+        },
+      },
+      cmdline_block = { { { 'if 1' } }, { { '  :let x = 1' } } },
+    })
+    feed('<CR>')
+    screen:expect({
+      grid = s1,
+      cmdline = { { abort = false } },
+    })
+  end)
 end
 
 -- the representation of cmdline and cmdline_block contents changed with ext_linegrid