fix(diagnostic): open_float() handles config.float function #31577

Problem:
The `float` field of vim.diagnostic.config can be a function,
but diagnostic.open_float() does not handle it correctly.

Solution:
Add handling for it in open_float().

runtime/doc/diagnostic.txt

@@ -545,7 +545,7 @@ Lua module: vim.diagnostic                                    *diagnostic-api*
                              Use virtual lines for diagnostics.
       • {signs}?             (`boolean|vim.diagnostic.Opts.Signs|fun(namespace: integer, bufnr:integer): vim.diagnostic.Opts.Signs`, default: `true`)
                              Use signs for diagnostics |diagnostic-signs|.
-      • {float}?             (`boolean|vim.diagnostic.Opts.Float|fun(namespace: integer, bufnr:integer): vim.diagnostic.Opts.Float`)
+      • {float}?             (`boolean|vim.diagnostic.Opts.Float|fun(namespace?: integer|integer[], bufnr:integer): vim.diagnostic.Opts.Float`)
                              Options for floating windows. See
                              |vim.diagnostic.Opts.Float|.
       • {status}?            (`vim.diagnostic.Opts.Status`) Options for the

runtime/lua/vim/diagnostic.lua

@@ -92,7 +92,7 @@ end
 --- @field signs? boolean|vim.diagnostic.Opts.Signs|fun(namespace: integer, bufnr:integer): vim.diagnostic.Opts.Signs
 ---
 --- Options for floating windows. See |vim.diagnostic.Opts.Float|.
---- @field float? boolean|vim.diagnostic.Opts.Float|fun(namespace: integer, bufnr:integer): vim.diagnostic.Opts.Float
+--- @field float? boolean|vim.diagnostic.Opts.Float|fun(namespace?: integer|integer[], bufnr:integer): vim.diagnostic.Opts.Float
 ---
 --- Options for the statusline component.
 --- @field status? vim.diagnostic.Opts.Status
@@ -2439,10 +2439,13 @@ function M.open_float(opts, ...)
     -- Resolve options with user settings from vim.diagnostic.config
     -- Unlike the other decoration functions (e.g. set_virtual_text, set_signs, etc.) `open_float`
     -- does not have a dedicated table for configuration options; instead, the options are mixed in
-    -- with its `opts` table which also includes "keyword" parameters. So we create a dedicated
-    -- options table that inherits missing keys from the global configuration before resolving.
-    local t = global_diagnostic_options.float
-    local float_opts = vim.tbl_extend('keep', opts, type(t) == 'table' and t or {})
+    -- with its `opts` table. We create a dedicated options table (`float_opts`) that inherits
+    -- missing keys from the global configuration (`global_diagnostic_options.float`), which can
+    -- be a table or a function.
+    local o = global_diagnostic_options
+    local t = type(o.float) == 'table' and o.float
+      or (type(o.float) == 'function' and o.float(opts.namespace, bufnr) or {})
+    local float_opts = vim.tbl_extend('keep', opts, t)
     opts = get_resolved_options({ float = float_opts }, nil, bufnr).float
   end
 

test/functional/lua/diagnostic_spec.lua

@@ -3215,28 +3215,40 @@ describe('vim.diagnostic', function()
       )
     end)
 
-    it(
-      'creates floating window and returns float bufnr and winnr without header, if requested',
-      function()
-        -- One line (since no header):
-        --    1. <msg>
-        eq(
-          1,
-          exec_lua(function()
-            local diagnostics = {
-              _G.make_error('Syntax error', 0, 1, 0, 3),
-            }
-            vim.api.nvim_win_set_buf(0, _G.diagnostic_bufnr)
-            vim.diagnostic.set(_G.diagnostic_ns, _G.diagnostic_bufnr, diagnostics)
-            local float_bufnr, winnr =
-              vim.diagnostic.open_float(_G.diagnostic_bufnr, { header = false })
-            local lines = vim.api.nvim_buf_get_lines(float_bufnr, 0, -1, false)
-            vim.api.nvim_win_close(winnr, true)
-            return #lines
-          end)
-        )
-      end
-    )
+    it('creates floating window without header and supports function-based float config', function()
+      -- One line (since no header):
+      --    1. <msg>
+      eq(
+        { 1, '╭', '┌' },
+        exec_lua(function()
+          local diagnostics = {
+            _G.make_error('Syntax error', 0, 1, 0, 3),
+          }
+          vim.api.nvim_win_set_buf(0, _G.diagnostic_bufnr)
+          vim.diagnostic.set(_G.diagnostic_ns, _G.diagnostic_bufnr, diagnostics)
+          local float_bufnr, winnr =
+            vim.diagnostic.open_float(_G.diagnostic_bufnr, { header = false })
+          local lines = vim.api.nvim_buf_get_lines(float_bufnr, 0, -1, false)
+          vim.api.nvim_win_close(winnr, true)
+          vim.diagnostic.config({
+            float = function(_, bufnr)
+              return { border = bufnr == _G.diagnostic_bufnr and 'rounded' or 'single' }
+            end,
+          })
+          _, winnr = vim.diagnostic.open_float()
+          local border1 = vim.api.nvim_win_get_config(winnr).border
+          vim.api.nvim_win_close(winnr, true)
+
+          local other_bufnr = vim.api.nvim_create_buf(true, true)
+          vim.api.nvim_win_set_buf(0, other_bufnr)
+          vim.diagnostic.set(_G.diagnostic_ns, other_bufnr, diagnostics)
+          local _, winnr1 = vim.diagnostic.open_float()
+          local border2 = vim.api.nvim_win_get_config(winnr1).border
+          vim.api.nvim_win_close(winnr1, true)
+          return { #lines, border1[1], border2[1] }
+        end)
+      )
+    end)
 
     it('clamps diagnostic line numbers within the valid range', function()
       eq(