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
@@ -63,8 +63,14 @@
 
   void verifyFormat(llvm::StringRef Code,
                     const FormatStyle &Style = getLLVMStyle()) {
-    EXPECT_EQ(Code.str(), format(Code, Style)) << "Expected code is not stable";
-    EXPECT_EQ(Code.str(), format(test::messUp(Code), Style));
+    verifyFormat(Code, Code, Style);
+  }
+
+  void verifyFormat(llvm::StringRef Expected, llvm::StringRef Code,
+                    const FormatStyle &Style = getLLVMStyle()) {
+    EXPECT_EQ(Expected.str(), format(Expected, Style))
+        << "Expected code is not stable";
+    EXPECT_EQ(Expected.str(), format(test::messUp(Code), Style));
   }
 
   void verifyGoogleFormat(llvm::StringRef Code) {
@@ -715,6 +721,11 @@
             "//: 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) {