fix(api): only flush nvim__redraw when necessary #31250

Problem:  Not possible to only set a "redraw later" type with
          nvim__redraw, which seems to be desired for the
          treesitter highlighter.
Solution: Do not update the screen when "flush" is explicitly set to
          false and only redraw later types are present. In that case,
          do not call ui_flush() either.

runtime/lua/vim/lsp/inlay_hint.lua

@@ -65,7 +65,7 @@ function M.on_inlayhint(err, result, ctx)
   if num_unprocessed == 0 then
     client_hints[client_id] = {}
     bufstate.version = ctx.version
-    api.nvim__redraw({ buf = bufnr, valid = true })
+    api.nvim__redraw({ buf = bufnr, valid = true, flush = false })
     return
   end
 
@@ -81,7 +81,7 @@ function M.on_inlayhint(err, result, ctx)
 
   client_hints[client_id] = new_lnum_hints
   bufstate.version = ctx.version
-  api.nvim__redraw({ buf = bufnr, valid = true })
+  api.nvim__redraw({ buf = bufnr, valid = true, flush = false })
 end
 
 --- |lsp-handler| for the method `workspace/inlayHint/refresh`
@@ -215,7 +215,7 @@ local function clear(bufnr)
     end
   end
   api.nvim_buf_clear_namespace(bufnr, namespace, 0, -1)
-  api.nvim__redraw({ buf = bufnr, valid = true })
+  api.nvim__redraw({ buf = bufnr, valid = true, flush = false })
 end
 
 --- Disable inlay hints for a buffer

runtime/lua/vim/treesitter/highlighter.lua

@@ -219,8 +219,8 @@ end
 ---@package
 ---@param changes Range6[]
 function TSHighlighter:on_changedtree(changes)
-  for i, ch in ipairs(changes) do
-    api.nvim__redraw({ buf = self.bufnr, range = { ch[1], ch[4] + 1 }, flush = i == #changes })
+  for _, ch in ipairs(changes) do
+    api.nvim__redraw({ buf = self.bufnr, range = { ch[1], ch[4] + 1 }, flush = false })
   end
 end
 

src/nvim/api/vim.c

@@ -2396,9 +2396,16 @@ void nvim__redraw(Dict(redraw) *opts, Error *err)
       last = rbuf->b_ml.ml_line_count;
     }
     redraw_buf_range_later(rbuf, first, last);
+  }
+
+  // Redraw later types require update_screen() so call implicitly unless set to false.
+  if (HAS_KEY(opts, redraw, valid) || HAS_KEY(opts, redraw, range)) {
     opts->flush = HAS_KEY(opts, redraw, flush) ? opts->flush : true;
   }
 
+  // When explicitly set to false and only "redraw later" types are present,
+  // don't call ui_flush() either.
+  bool flush_ui = opts->flush;
   if (opts->tabline) {
     // Flush later in case tabline was just hidden or shown for the first time.
     if (redraw_tabline && firstwin->w_lines_valid == 0) {
@@ -2406,6 +2413,7 @@ void nvim__redraw(Dict(redraw) *opts, Error *err)
     } else {
       draw_tabline();
     }
+    flush_ui = true;
   }
 
   bool save_lz = p_lz;
@@ -2422,6 +2430,7 @@ void nvim__redraw(Dict(redraw) *opts, Error *err)
     } else {
       redraw_status(win, opts, &opts->flush);
     }
+    flush_ui = true;
   }
 
   win_T *cwin = win ? win : curwin;
@@ -2438,9 +2447,12 @@ void nvim__redraw(Dict(redraw) *opts, Error *err)
 
   if (opts->cursor) {
     setcursor_mayforce(cwin, true);
+    flush_ui = true;
   }
 
-  ui_flush();
+  if (flush_ui) {
+    ui_flush();
+  }
 
   RedrawingDisabled = save_rd;
   p_lz = save_lz;