Logic move to CellColorResolver

src/common/buffer/AttributeData.ts

@@ -127,9 +127,7 @@ export class AttributeData implements IAttributeData {
       : UnderlineStyle.NONE;
   }
   public getUnderlineVarinatOffset(): number {
-    return this.fg & FgFlags.UNDERLINE
-      ? (this.bg & BgFlags.HAS_EXTENDED ? this.extended.underlineVarinatOffset : 0)
-      : 0;
+    return this.extended.underlineVarinatOffset;
   }
 }
 
@@ -180,7 +178,11 @@ export class ExtendedAttrs implements IExtendedAttrs {
   }
 
   public get underlineVarinatOffset(): number {
-    return (this._ext & ExtFlags.VARIANT_OFFSET) >> 29;
+    const val = (this._ext & ExtFlags.VARIANT_OFFSET) >> 29;
+    if (val < 0) {
+      return val ^ 4294967288;
+    }
+    return val;
   }
   public set underlineVarinatOffset(value: number) {
     this._ext &= ~ExtFlags.VARIANT_OFFSET;

src/common/buffer/BufferLine.test.ts

@@ -119,6 +119,18 @@ describe('AttributeData', () => {
       attrs.fg &= ~FgFlags.UNDERLINE;
       assert.equal(attrs.getUnderlineStyle(), UnderlineStyle.NONE);
     });
+    it('getUnderlineVarinatOffset', () => {
+      const attrs = new AttributeData();
+
+      // defaults to no offset
+      assert.equal(attrs.getUnderlineVarinatOffset(), 0);
+
+      // should return 0 - 7
+      for (let i = 0; i < 8; ++i) {
+        attrs.extended.underlineVarinatOffset = i;
+        assert.equal(attrs.getUnderlineVarinatOffset(), i);
+      }
+    });
   });
 });