Index: clang/lib/Format/BreakableToken.cpp
===================================================================
--- clang/lib/Format/BreakableToken.cpp
+++ clang/lib/Format/BreakableToken.cpp
@@ -82,9 +82,9 @@
        NumChars < MaxSplit && MaxSplitBytes < Text.size();) {
     unsigned BytesInChar =
         encoding::getCodePointNumBytes(Text[MaxSplitBytes], Encoding);
-    NumChars +=
-        encoding::columnWidthWithTabs(Text.substr(MaxSplitBytes, BytesInChar),
-                                      ContentStartColumn, TabWidth, Encoding);
+    NumChars += encoding::columnWidthWithTabs(
+        Text.substr(MaxSplitBytes, BytesInChar), ContentStartColumn + NumChars,
+        TabWidth, Encoding);
     MaxSplitBytes += BytesInChar;
   }
 
Index: clang/unittests/Format/FormatTestComments.cpp
===================================================================
--- clang/unittests/Format/FormatTestComments.cpp
+++ clang/unittests/Format/FormatTestComments.cpp
@@ -729,6 +729,11 @@
             "//: one line",
             format("//: A comment that doesn't fit on one line",
                    getLLVMStyleWithColumns(20)));
+
+  verifyFormat(
+      "//\t\t\t\tofMap(message.velocity, 0, 127, 0, ofGetWidth()\n"
+      "//* 0.2)",
+      "//\t\t\t\tofMap(message.velocity, 0, 127, 0, ofGetWidth() * 0.2)");
 }
 
 TEST_F(FormatTestComments, PreservesHangingIndentInCxxComments) {