fix(lsp): codelens text flickers #38782

Problem:
When a new textDocument/codeLens response arrives with unresolved lenses,
on_win clears the existing codelens row before codeLens/resolve
completes. This causes the displayed codelens text to flicker while
typing.

Solution:
Keep the current virtual lines if any of the refreshed lenses are still
unresolved. Clear the old virtual lines only when the line no longer has
lenses or all its lenses are resolved.

A trade-off is that the user may temporarily see outdated codelenses.
However, that's preferable to spamming updates on every refresh.

AI-assisted: Codex

runtime/lua/vim/lsp/codelens.lua

@@ -227,11 +227,10 @@ function Provider:on_win(toprow, botrow)
       for client_id, state in pairs(self.client_state) do
         local bufnr = self.bufnr
         local namespace = state.namespace
-
-        api.nvim_buf_clear_namespace(bufnr, namespace, row, row + 1)
-
         local lenses = state.row_lenses[row]
-        if lenses then
+        if not lenses then
+          api.nvim_buf_clear_namespace(bufnr, namespace, row, row + 1)
+        else
           table.sort(lenses, function(a, b)
             return a.range.start.character < b.range.start.character
           end)
@@ -242,10 +241,12 @@ function Provider:on_win(toprow, botrow)
           local virt_text = {
             { string.rep(' ', range.start_col), 'LspCodeLensSeparator' },
           }
+          local has_unresolved = false
 
           for _, lens in ipairs(lenses) do
             -- A code lens is unresolved when no command is associated to it.
             if not lens.command then
+              has_unresolved = true
               self:resolve(client, lens)
             else
               vim.list_extend(virt_text, {
@@ -254,25 +255,37 @@ function Provider:on_win(toprow, botrow)
               })
             end
           end
-          -- Remove trailing separator.
-          table.remove(virt_text)
 
-          -- Use a placeholder to prevent flickering caused by layout shifts.
-          if #virt_text == 1 then
-            table.insert(virt_text, { '', 'LspCodeLens' })
-          end
+          local had_extmark = #api.nvim_buf_get_extmarks(
+            bufnr,
+            namespace,
+            { row, 0 },
+            { row, -1 },
+            {}
+          ) > 0
 
-          api.nvim_buf_set_extmark(bufnr, namespace, row, 0, {
-            virt_lines = { virt_text },
-            virt_lines_above = true,
-            virt_lines_overflow = 'scroll',
-            hl_mode = 'combine',
-          })
+          if not has_unresolved or not had_extmark then
+            -- Remove trailing separator.
+            table.remove(virt_text)
 
-          -- Fix https://github.com/neovim/neovim/issues/16166
-          -- Make sure the code lens on the first line is visible when updating.
-          if row == 0 then
-            vim.fn.winrestview({ topfill = 1 })
+            -- Use a placeholder to prevent flickering caused by layout shifts.
+            if #virt_text == 1 then
+              table.insert(virt_text, { '', 'LspCodeLens' })
+            end
+
+            api.nvim_buf_clear_namespace(bufnr, namespace, row, row + 1)
+            api.nvim_buf_set_extmark(bufnr, namespace, row, 0, {
+              virt_lines = { virt_text },
+              virt_lines_above = true,
+              virt_lines_overflow = 'scroll',
+              hl_mode = 'combine',
+            })
+
+            -- Fix https://github.com/neovim/neovim/issues/16166
+            -- Make sure the code lens on the first line is visible when updating.
+            if row == 0 then
+              vim.fn.winrestview({ topfill = 1 })
+            end
           end
         end
         self.row_version[row] = self.version