fix(highlight): blend_alpha controls line bg and prefix matches gutter (#178)

doc/diffs.nvim.txt

@@ -241,11 +241,13 @@ Configuration is done via `vim.g.diffs`. Set this before the plugin loads:
                              Only visible if line numbers are enabled.
 
         {blend_alpha}        (number, default: 0.6)
-                             Alpha value for character-level blend intensity.
+                             Alpha value for diff line background intensity.
-                             Controls how strongly changed characters stand
+                             Controls how strongly diff lines (adds/deletes)
-                             out from the line-level background. Must be
+                             stand out from the editor background. Intra-line
-                             between 0 and 1 (inclusive). Higher values
+                             highlights use alpha + 0.3 (capped at 1.0) for
-                             produce more vivid character-level highlights.
+                             extra contrast. Must be between 0 and 1
+                             (inclusive). Higher values produce more vivid
+                             backgrounds.
 
         {context}            (table, default: see below)
                              Syntax parsing context options.

lua/diffs/highlight.lua

@@ -612,6 +612,12 @@ function M.highlight_hunk(bufnr, ns, hunk, opts)
             })
           end
         end
+      elseif opts.highlights.background and is_diff_line then
+        pcall(vim.api.nvim_buf_set_extmark, bufnr, ns, buf_line, 0, {
+          end_col = 1,
+          hl_group = number_hl,
+          priority = p.syntax,
+        })
       end
 
       if opts.highlights.background and is_diff_line then

lua/diffs/init.lua

@@ -522,28 +522,17 @@ local function compute_highlight_groups(is_default)
   local normal_fg = normal.fg or (dark and 0xcccccc or 0x333333)
 
   local alpha = config.highlights.blend_alpha or 0.6
-  local blended_add, blended_del, blended_add_text, blended_del_text
+  local text_alpha = math.min(alpha + 0.3, 1.0)
-  if transparent then
+  local blended_add = blend_color(add_bg, bg, alpha)
-    blended_add = add_bg
+  local blended_del = blend_color(del_bg, bg, alpha)
-    blended_del = del_bg
+  local blended_add_text = blend_color(add_bg, bg, text_alpha)
-    blended_add_text = blend_color(add_fg, bg, alpha)
+  local blended_del_text = blend_color(del_bg, bg, text_alpha)
-    blended_del_text = blend_color(del_fg, bg, alpha)
-  else
-    blended_add = blend_color(add_bg, bg, 0.4)
-    blended_del = blend_color(del_bg, bg, 0.4)
-    blended_add_text = blend_color(add_fg, bg, alpha)
-    blended_del_text = blend_color(del_fg, bg, alpha)
-  end
 
-  vim.api.nvim_set_hl(0, 'DiffsClear', { default = dflt, fg = normal_fg, bg = normal.bg })
+  vim.api.nvim_set_hl(0, 'DiffsClear', { default = dflt, fg = normal_fg, bg = bg })
   vim.api.nvim_set_hl(0, 'DiffsAdd', { default = dflt, bg = blended_add })
   vim.api.nvim_set_hl(0, 'DiffsDelete', { default = dflt, bg = blended_del })
-  vim.api.nvim_set_hl(0, 'DiffsAddNr', { default = dflt, fg = blended_add_text, bg = blended_add })
+  vim.api.nvim_set_hl(0, 'DiffsAddNr', { default = dflt, fg = add_fg, bg = blended_add })
-  vim.api.nvim_set_hl(
+  vim.api.nvim_set_hl(0, 'DiffsDeleteNr', { default = dflt, fg = del_fg, bg = blended_del })
-    0,
-    'DiffsDeleteNr',
-    { default = dflt, fg = blended_del_text, bg = blended_del }
-  )
   vim.api.nvim_set_hl(0, 'DiffsAddText', { default = dflt, bg = blended_add_text })
   vim.api.nvim_set_hl(0, 'DiffsDeleteText', { default = dflt, bg = blended_del_text })
 
@@ -577,23 +566,13 @@ local function compute_highlight_groups(is_default)
   local text_bg = diff_text.bg or 0x4a4a5a
   local change_fg = diff_change.fg or diff_text.fg or 0x80a0c0
 
-  local blended_ours, blended_theirs, blended_base
+  local base_alpha = math.max(alpha - 0.1, 0.0)
-  local blended_ours_nr, blended_theirs_nr, blended_base_nr
+  local blended_ours = blend_color(add_bg, bg, alpha)
-  if transparent then
+  local blended_theirs = blend_color(change_bg, bg, alpha)
-    blended_ours = add_bg
+  local blended_base = blend_color(text_bg, bg, base_alpha)
-    blended_theirs = change_bg
+  local blended_ours_nr = add_fg
-    blended_base = text_bg
+  local blended_theirs_nr = change_fg
-    blended_ours_nr = blend_color(add_fg, bg, alpha)
+  local blended_base_nr = change_fg
-    blended_theirs_nr = blend_color(change_fg, bg, alpha)
-    blended_base_nr = blend_color(change_fg, bg, 0.4)
-  else
-    blended_ours = blend_color(add_bg, bg, 0.4)
-    blended_theirs = blend_color(change_bg, bg, 0.4)
-    blended_base = blend_color(text_bg, bg, 0.3)
-    blended_ours_nr = blend_color(add_fg, bg, alpha)
-    blended_theirs_nr = blend_color(change_fg, bg, alpha)
-    blended_base_nr = blend_color(change_fg, bg, 0.4)
-  end
 
   vim.api.nvim_set_hl(0, 'DiffsConflictOurs', { default = dflt, bg = blended_ours })
   vim.api.nvim_set_hl(0, 'DiffsConflictTheirs', { default = dflt, bg = blended_theirs })
@@ -1145,6 +1124,9 @@ M._test = {
   set_hl_retry_pending = function(v)
     hl_retry_pending = v
   end,
+  get_config = function()
+    return config
+  end,
 }
 
 return M

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