fix: incsearch flickering when typing characters during search

Setting the decoration provider on every renderer:draw() command caused
flickering of incsearch highlights when showing completions for / and ?
search modes on the cmdline as characters were typed.

Decoration providers only need to be set once and they remain active, so
the fix here is create it when the renderer is created and remember the
last bufnr that was drawn to, along with the columns.

Fixes #1061

lua/blink/cmp/completion/windows/render/init.lua

@@ -3,6 +3,7 @@
 --- @field padding number[]
 --- @field gap number
 --- @field columns blink.cmp.DrawColumn[]
+--- @field bufnr number?
 ---
 --- @field new fun(draw: blink.cmp.Draw): blink.cmp.Renderer
 --- @field draw fun(self: blink.cmp.Renderer, context: blink.cmp.Context, bufnr: number, items: blink.cmp.CompletionItem[], draw: blink.cmp.Draw): blink.cmp.DrawColumn[]
@@ -25,6 +26,35 @@ function renderer.new(draw)
   self.padding = padding
   self.gap = draw.gap
   self.def = draw
+
+  -- Setting highlights is slow and we update on every keystroke so we instead use a decoration provider
+  -- which will only render highlights of the visible lines. This also avoids having to do virtual scroll
+  -- like nvim-cmp does, which breaks on UIs like neovide
+  vim.api.nvim_set_decoration_provider(ns, {
+    on_win = function(_, _, win_bufnr) return self.bufnr == win_bufnr end,
+    on_line = function(_, _, _, line)
+      local offset = self.padding[1]
+      for _, column in ipairs(self.columns) do
+        local text = column:get_line_text(line + 1)
+        if #text > 0 then
+          local highlights = column:get_line_highlights(line + 1)
+          for _, highlight in ipairs(highlights) do
+            local col = offset + highlight[1]
+            local end_col = offset + highlight[2]
+            vim.api.nvim_buf_set_extmark(self.bufnr, ns, line, col, {
+              end_col = end_col,
+              hl_group = highlight.group,
+              hl_mode = 'combine',
+              hl_eol = true,
+              ephemeral = true,
+            })
+          end
+          offset = offset + #text + self.gap
+        end
+      end
+    end,
+  })
+
   return self
 end
 
@@ -88,35 +118,8 @@ function renderer:draw(context, bufnr, items)
   vim.api.nvim_buf_set_lines(bufnr, 0, -1, false, lines)
   vim.api.nvim_set_option_value('modified', false, { buf = bufnr })
 
-  -- Setting highlights is slow and we update on every keystroke so we instead use a decoration provider
-  -- which will only render highlights of the visible lines. This also avoids having to do virtual scroll
-  -- like nvim-cmp does, which breaks on UIs like neovide
-  vim.api.nvim_set_decoration_provider(ns, {
-    on_win = function(_, _, win_bufnr) return bufnr == win_bufnr end,
-    on_line = function(_, _, _, line)
-      local offset = self.padding[1]
-      for _, column in ipairs(columns) do
-        local text = column:get_line_text(line + 1)
-        if #text > 0 then
-          local highlights = column:get_line_highlights(line + 1)
-          for _, highlight in ipairs(highlights) do
-            local col = offset + highlight[1]
-            local end_col = offset + highlight[2]
-            vim.api.nvim_buf_set_extmark(bufnr, ns, line, col, {
-              end_col = end_col,
-              hl_group = highlight.group,
-              hl_mode = 'combine',
-              hl_eol = true,
-              ephemeral = true,
-            })
-          end
-          offset = offset + #text + self.gap
-        end
-      end
-    end,
-  })
-
   self.columns = columns
+  self.bufnr = bufnr
 end
 
 function renderer:get_component_column_location(columns, component_name)