fix(api): nvim_set_hl boolean false corrupts underline styles (#38504)

Problem: setting one underline style to false clears bits belonging
to another style. `{underdouble = true, underdashed = false}` results
in undercurl because CHECK_FLAG_WITH_KEY does `m &= ~flag` which
doesn't work for multi-bit encoded values sharing HL_UNDERLINE_MASK.

Solution: use a local variable to derive the correct clear mask from
the flag. Clear the whole HL_UNDERLINE_MASK field instead of individual
bits, and only clear on false when the current style actually matches.

src/nvim/highlight.c

@@ -1039,13 +1039,12 @@ HlAttrs dict2hlattrs(Dict(highlight) *dict, bool use_rgb, int *link_id, HlAttrs
 
 #define CHECK_FLAG_WITH_KEY(d, m, name, extra, flag) \
   if (HAS_KEY_X(d, name)) { \
+    int32_t flag_ = (flag); \
+    int32_t cmask_ = (flag_ & HL_UNDERLINE_MASK) ? HL_UNDERLINE_MASK : flag_; \
     if (d->name##extra) { \
-      if (flag & HL_UNDERLINE_MASK) { \
+      m = (m & ~cmask_) | flag_; \
-        m &= ~HL_UNDERLINE_MASK; \
+    } else if ((m & cmask_) == flag_) { \
-      } \
+      m &= ~cmask_; \
-      m |= flag; \
-    } else { \
-      m &= ~flag; \
     } \
   }
 

test/functional/api/highlight_spec.lua

@@ -290,6 +290,39 @@ describe('API: set highlight', function()
     hl = api.nvim_get_hl(0, { name = 'LinkedGroup' })
     eq(nil, hl.link)
     eq(true, hl.bold)
+
+    -- underline style flags: false must not corrupt other styles
+    local unders = { 'underline', 'undercurl', 'underdouble', 'underdotted', 'underdashed' }
+    for _, a in ipairs(unders) do
+      for _, b in ipairs(unders) do
+        if a ~= b then
+          api.nvim_set_hl(0, 'TestGroup', { [a] = true, [b] = false })
+          hl = api.nvim_get_hl(0, { name = 'TestGroup' })
+          eq(true, hl[a])
+          eq(nil, hl[b])
+        end
+      end
+    end
+    api.nvim_set_hl(0, 'TestGroup', { underdouble = true, fg = '#ff0000', bold = true })
+    api.nvim_set_hl(0, 'TestGroup', { fg = '#00ff00', update = true })
+    hl = api.nvim_get_hl(0, { name = 'TestGroup' })
+    eq(true, hl.underdouble)
+    eq(true, hl.bold)
+    eq(65280, hl.fg)
+
+    api.nvim_set_hl(0, 'TestGroup', { underdashed = true, update = true })
+    hl = api.nvim_get_hl(0, { name = 'TestGroup' })
+    eq(true, hl.underdashed)
+    eq(nil, hl.underdouble)
+
+    api.nvim_set_hl(0, 'TestGroup', { underdouble = true, bold = true })
+    api.nvim_set_hl(0, 'TestGroup', { underdashed = false, update = true })
+    hl = api.nvim_get_hl(0, { name = 'TestGroup' })
+    eq(true, hl.underdouble)
+    api.nvim_set_hl(0, 'TestGroup', { underdouble = false, update = true })
+    hl = api.nvim_get_hl(0, { name = 'TestGroup' })
+    eq(nil, hl.underdouble)
+    eq(true, hl.bold)
   end)
 end)