fix(ui): avoid ambiguity about last chunk when flushing halfway (#29718)

src/nvim/api/ui.c

@@ -778,16 +778,26 @@ void remote_ui_raw_line(RemoteUI *ui, Integer grid, Integer row, Integer startco
     for (size_t i = 0; i < ncells; i++) {
       repeat++;
       if (i == ncells - 1 || attrs[i] != attrs[i + 1] || chunk[i] != chunk[i + 1]) {
-        if (UI_BUF_SIZE - BUF_POS(ui) < 2 * (1 + 2 + MAX_SCHAR_SIZE + 5 + 5) + 1
+        if (
+            // Close to overflowing the redraw buffer. Finish this event, flush,
+            // and start a new "grid_line" event at the current position.
+            // For simplicity leave place for the final "clear" element as well,
+            // hence the factor of 2 in the check.
+            UI_BUF_SIZE - BUF_POS(ui) < 2 * (1 + 2 + MAX_SCHAR_SIZE + 5 + 5) + 1
+            // Also if there is a lot of packed cells, pass them off to the UI to
+            // let it start processing them.
             || ui->ncells_pending >= 500) {
-          // close to overflowing the redraw buffer. finish this event,
-          // flush, and start a new "grid_line" event at the current position.
-          // For simplicity leave place for the final "clear" element
-          // as well, hence the factor of 2 in the check.
-          // Also if there is a lot of packed cells, pass them of to the UI to
-          // let it start processing them
+          // If the last chunk was all spaces, add an empty clearing chunk,
+          // so it's clear that the last chunk wasn't a clearing chunk.
+          if (was_space) {
+            nelem++;
+            ui->ncells_pending += 1;
+            mpack_array(buf, 3);
+            mpack_str_small(buf, S_LEN(" "));
+            mpack_uint(buf, (uint32_t)clearattr);
+            mpack_uint(buf, 0);
+          }
           mpack_w2(&lenpos, nelem);
-
           // We only ever set the wrap field on the final "grid_line" event for the line.
           mpack_bool(buf, false);
           ui_flush_buf(ui);

test/functional/terminal/testutil.lua

@@ -169,7 +169,7 @@ local function setup_child_nvim(args, opts)
     env.VIMRUNTIME = os.getenv('VIMRUNTIME')
   end
 
-  return screen_setup(0, argv, opts.cols, env)
+  return screen_setup(opts.extra_rows, argv, opts.cols, env)
 end
 
 return {

test/functional/terminal/tui_spec.lua

@@ -2199,6 +2199,47 @@ describe('TUI', function()
     }
   end)
 
+  it('draws screen lines with leading spaces correctly #29711', function()
+    local screen = tt.setup_child_nvim({
+      '-u',
+      'NONE',
+      '-i',
+      'NONE',
+      '--cmd',
+      'set foldcolumn=6 | call setline(1, ["", repeat("aabb", 1000)]) | echo 42',
+    }, { extra_rows = 10, cols = 66 })
+    screen:expect {
+      grid = [[
+                                                                        |
+            aabbaabbaabbaabbaabbaabbaabbaabbaabbaabbaabbaabbaabbaabbaabb|*12
+            aabbaabbaabbaabbaabbaabbaabbaabbaabbaabbaabbaabbaabbaabba@@@|
+      [No Name] [+]                                   1,0-1          Top|
+      42                                                                |
+      -- TERMINAL --                                                    |
+    ]],
+      attr_ids = {},
+    }
+    feed_data('\12') -- Ctrl-L
+    -- The first line counts as 3 cells.
+    -- For the second line, 6 repeated spaces at the start counts as 2 cells,
+    -- so each screen line of the second line counts as 62 cells.
+    -- After drawing the first line and 8 screen lines of the second line,
+    -- 3 + 8 * 62 = 499 cells have been counted.
+    -- The 6 repeated spaces at the start of the next screen line exceeds the
+    -- 500-cell limit, so the buffer is flushed after these spaces.
+    screen:expect {
+      grid = [[
+                                                                        |
+            aabbaabbaabbaabbaabbaabbaabbaabbaabbaabbaabbaabbaabbaabbaabb|*12
+            aabbaabbaabbaabbaabbaabbaabbaabbaabbaabbaabbaabbaabbaabba@@@|
+      [No Name] [+]                                   1,0-1          Top|
+                                                                        |
+      -- TERMINAL --                                                    |
+    ]],
+      attr_ids = {},
+    }
+  end)
+
   it('no heap-buffer-overflow when changing &columns', function()
     -- Set a different bg colour and change $TERM to something dumber so the `print_spaces()`
     -- codepath in `clear_region()` is hit.