add example/form.lua

lua/u/renderer.lua

@@ -296,10 +296,15 @@ function Renderer:_reconcile() -- {{{
 
   --
   -- Step 2: reconcile extmarks:
+  -- - You may be tempted to try to keep track of which extmarks are needed,
+  --   and only delete those that are not needed. However, each time a tree is
+  --   rendered, brand new extmarks are created. For simplicity, it is better to
+  --   just delete all extmarks, and recreate them.
   --
 
   -- Clear current extmarks:
   vim.api.nvim_buf_clear_namespace(self.bufnr, self.ns, 0, -1)
+
   -- Set current extmarks:
   for _, extmark in ipairs(self.curr.extmarks) do
     extmark.id = vim.api.nvim_buf_set_extmark(