[continuecomment] skip some edge cases, respect indentWithTabs

* line comment:
  - if both cursor and line comment are at pos 0
  - if cursor is at EOL or in trailing space at EOL, but always continue if the next line is a line comment
* block comment:
  - if it's inside a line comment e.g. // /*

Author: tophf

addon/comment/continuecomment.js

@@ -9,6 +9,8 @@
   else // Plain browser env
     mod(CodeMirror);
 })(function(CodeMirror) {
+  var nonspace = /\S/g;
+  var repeat = String.prototype.repeat || function (n) { return Array(n + 1).join(this); };
   function continueComment(cm) {
     if (cm.getOption("disableInput")) return CodeMirror.Pass;
     var ranges = cm.listSelections(), mode, inserts = [];
@@ -19,29 +21,56 @@
       if (!mode) mode = modeHere;
       else if (mode != modeHere) return CodeMirror.Pass;
 
-      var insert = null;
-      if (mode.blockCommentStart && mode.blockCommentContinue) {
-        var line = cm.getLine(pos.line).slice(0, pos.ch)
-        var end = line.lastIndexOf(mode.blockCommentEnd), found
-        if (end != -1 && end == pos.ch - mode.blockCommentEnd.length) {
-          // Comment ended, don't continue it
-        } else if ((found = line.lastIndexOf(mode.blockCommentStart)) > -1 && found > end) {
-          insert = line.slice(0, found)
-          if (/\S/.test(insert)) {
-            insert = ""
-            for (var j = 0; j < found; ++j) insert += " "
+      var insert = null, line, found;
+      var blockStart = mode.blockCommentStart, lineCmt = mode.lineComment;
+      if (blockStart && mode.blockCommentContinue) {
+        line = cm.getLine(pos.line);
+        var end = line.lastIndexOf(mode.blockCommentEnd, pos.ch - mode.blockCommentEnd.length);
+        // 1. if this block comment ended
+        // 2. if this is actually inside a line comment
+        if (end != -1 && end == pos.ch - mode.blockCommentEnd.length ||
+            lineCmt && (found = line.lastIndexOf(lineCmt, pos.ch - 1)) > -1 &&
+            /\bcomment\b/.test(cm.getTokenTypeAt({line: pos.line, ch: found + 1}))) {
+          // ...then don't continue it
+        } else if ((found = line.lastIndexOf(blockStart, pos.ch - blockStart.length)) > -1 &&
+                   found > end) {
+          // reuse the existing leading spaces/tabs/mixed
+          // or build the correct indent using CM's tab/indent options
+          if (nonspaceAfter(0, line) >= found) {
+            insert = line.slice(0, found);
+          } else {
+            var tabSize = cm.options.tabSize, numTabs;
+            found = CodeMirror.countColumn(line, found, tabSize);
+            insert = !cm.options.indentWithTabs ? repeat.call(" ", found) :
+              repeat.call("\t", (numTabs = Math.floor(found / tabSize))) +
+              repeat.call(" ", found - tabSize * numTabs);
           }
-        } else if ((found = line.indexOf(mode.blockCommentContinue)) > -1 && !/\S/.test(line.slice(0, found))) {
-          insert = line.slice(0, found)
+        } else if ((found = line.indexOf(mode.blockCommentContinue)) > -1 &&
+                   found <= pos.ch &&
+                   found <= nonspaceAfter(0, line)) {
+          insert = line.slice(0, found);
         }
         if (insert != null) insert += mode.blockCommentContinue
       }
-      if (insert == null && mode.lineComment && continueLineCommentEnabled(cm)) {
-        var line = cm.getLine(pos.line), found = line.indexOf(mode.lineComment);
-        if (found > -1) {
-          insert = line.slice(0, found);
-          if (/\S/.test(insert)) insert = null;
-          else insert += mode.lineComment + line.slice(found + mode.lineComment.length).match(/^\s*/)[0];
+      if (insert == null && lineCmt && continueLineCommentEnabled(cm)) {
+        if (line == null) line = cm.getLine(pos.line);
+        found = line.indexOf(lineCmt);
+        // cursor at pos 0, line comment also at pos 0 => shift it down, don't continue
+        if (!pos.ch && !found) insert = "";
+        // continue only if the line starts with an optional space + line comment
+        else if (found > -1 && nonspaceAfter(0, line) >= found) {
+          // don't continue if there's only space(s) after cursor or the end of the line
+          insert = nonspaceAfter(pos.ch, line) > -1;
+          // but always continue if the next line starts with a line comment too
+          if (!insert) {
+            var next = cm.getLine(pos.line + 1) || '',
+                nextFound = next.indexOf(lineCmt);
+            insert = nextFound > -1 && nonspaceAfter(0, next) >= nextFound || null;
+          }
+          if (insert) {
+            insert = line.slice(0, found) + lineCmt +
+                     line.slice(found + lineCmt.length).match(/^\s*/)[0];
+          }
         }
       }
       if (insert == null) return CodeMirror.Pass;
@@ -54,6 +83,12 @@
     });
   }
 
+  function nonspaceAfter(ch, str) {
+    nonspace.lastIndex = ch;
+    var m = nonspace.exec(str);
+    return m ? m.index : -1;
+  }
+
   function continueLineCommentEnabled(cm) {
     var opt = cm.getOption("continueComments");
     if (opt && typeof opt == "object")