test: add blend_alpha and prefix extmark coverage

spec/highlight_spec.lua

@@ -511,6 +511,83 @@ describe('highlight', function()
       delete_buffer(bufnr)
     end)
 
+    it('applies DiffsAddNr prefix extmark on + line for pw=1', function()
+      local bufnr = create_buffer({
+        '@@ -1,2 +1,2 @@',
+        '-old',
+        '+new',
+      })
+
+      local hunk = {
+        filename = 'test.lua',
+        lang = 'lua',
+        start_line = 1,
+        lines = { '-old', '+new' },
+        prefix_width = 1,
+      }
+
+      highlight.highlight_hunk(
+        bufnr,
+        ns,
+        hunk,
+        default_opts({ highlights = { background = true, treesitter = { enabled = false } } })
+      )
+
+      local extmarks = get_extmarks(bufnr)
+      local add_prefix = false
+      local del_prefix = false
+      for _, mark in ipairs(extmarks) do
+        local d = mark[4]
+        if d and d.end_col == 1 and mark[3] == 0 then
+          if d.hl_group == 'DiffsAddNr' and mark[2] == 2 then
+            add_prefix = true
+          end
+          if d.hl_group == 'DiffsDeleteNr' and mark[2] == 1 then
+            del_prefix = true
+          end
+        end
+      end
+      assert.is_true(add_prefix, 'DiffsAddNr on + prefix')
+      assert.is_true(del_prefix, 'DiffsDeleteNr on - prefix')
+      delete_buffer(bufnr)
+    end)
+
+    it('does not apply prefix extmark on context line', function()
+      local bufnr = create_buffer({
+        '@@ -1,2 +1,2 @@',
+        ' ctx',
+        '+new',
+      })
+
+      local hunk = {
+        filename = 'test.lua',
+        lang = 'lua',
+        start_line = 1,
+        lines = { ' ctx', '+new' },
+        prefix_width = 1,
+      }
+
+      highlight.highlight_hunk(
+        bufnr,
+        ns,
+        hunk,
+        default_opts({ highlights = { background = true, treesitter = { enabled = false } } })
+      )
+
+      local extmarks = get_extmarks(bufnr)
+      local ctx_prefix = false
+      for _, mark in ipairs(extmarks) do
+        local d = mark[4]
+        if d and mark[2] == 1 and mark[3] == 0 and d.end_col == 1 then
+          if d.hl_group == 'DiffsAddNr' or d.hl_group == 'DiffsDeleteNr' then
+            ctx_prefix = true
+          end
+        end
+      end
+      assert.is_false(ctx_prefix, 'no prefix extmark on context line')
+      delete_buffer(bufnr)
+    end)
+
     it('applies vim syntax extmarks when vim.enabled and no TS parser', function()
       local orig_synID = vim.fn.synID
       local orig_synIDtrans = vim.fn.synIDtrans

spec/init_spec.lua

@@ -554,7 +554,7 @@ describe('diffs', function()
       diffs._test.set_hl_retry_pending(false)
     end)
 
-    it('omits DiffsClear.bg when Normal.bg is nil (transparent)', function()
+    it('uses dark fallback bg for DiffsClear when Normal.bg is nil (transparent)', function()
       vim.api.nvim_get_hl = function(ns, opts)
         if opts.name == 'Normal' then
           return { fg = 0xc0c0c0 }
@@ -562,11 +562,38 @@ describe('diffs', function()
         return saved_get_hl(ns, opts)
       end
       diffs._test.compute_highlight_groups()
-      assert.is_nil(set_calls.DiffsClear.bg)
+      assert.are.equal(0x1a1a1a, set_calls.DiffsClear.bg)
       assert.is_table(set_calls.DiffsAdd)
       assert.is_table(set_calls.DiffsDelete)
     end)
 
+    it('blend_alpha controls DiffsAdd.bg intensity', function()
+      local saved_config_alpha = diffs._test.get_config().highlights.blend_alpha
+      diffs._test.get_config().highlights.blend_alpha = 0.3
+      vim.api.nvim_get_hl = function(ns, opts)
+        if opts.name == 'Normal' then
+          return { fg = 0xc0c0c0, bg = 0x1e1e2e }
+        end
+        if opts.name == 'DiffAdd' then
+          return { bg = 0x1a3a1a }
+        end
+        if opts.name == 'DiffDelete' then
+          return { bg = 0x3a1a1a }
+        end
+        return saved_get_hl(ns, opts)
+      end
+      diffs._test.compute_highlight_groups()
+      local bg_03 = set_calls.DiffsAdd.bg
+
+      diffs._test.get_config().highlights.blend_alpha = 0.9
+      diffs._test.compute_highlight_groups()
+      local bg_09 = set_calls.DiffsAdd.bg
+
+      assert.is_not.equal(bg_03, bg_09)
+
+      diffs._test.get_config().highlights.blend_alpha = saved_config_alpha
+    end)
+
     it('retries once then stops when Normal.bg stays nil', function()
       vim.api.nvim_get_hl = function(ns, opts)
         if opts.name == 'Normal' then