[lint addon] Use center of target node rather than mouse pos to find inline annotations

Since the mouseover will necessarily happen at the very edge of the node, and thus
give ambiguous coordinates.

Closes #2821

addon/lint/lint.js

@@ -171,20 +171,14 @@
     showTooltipFor(e, annotationTooltip(ann), target);
   }
 
-  // When the mouseover fires, the cursor might not actually be over
-  // the character itself yet. These pairs of x,y offsets are used to
-  // probe a few nearby points when no suitable marked range is found.
-  var nearby = [0, 0, 0, 5, 0, -5, 5, 0, -5, 0];
-
   function onMouseOver(cm, e) {
-    if (!/\bCodeMirror-lint-mark-/.test((e.target || e.srcElement).className)) return;
-    for (var i = 0; i < nearby.length; i += 2) {
-      var spans = cm.findMarksAt(cm.coordsChar({left: e.clientX + nearby[i],
-                                                top: e.clientY + nearby[i + 1]}, "client"));
-      for (var j = 0; j < spans.length; ++j) {
-        var span = spans[j], ann = span.__annotation;
-        if (ann) return popupSpanTooltip(ann, e);
-      }
+    var target = e.target || e.srcElement;
+    if (!/\bCodeMirror-lint-mark-/.test(target.className)) return;
+    var box = target.getBoundingClientRect(), x = (box.left + box.right) / 2, y = (box.top + box.bottom) / 2;
+    var spans = cm.findMarksAt(cm.coordsChar({left: x, top: y}, "client"));
+    for (var i = 0; i < spans.length; ++i) {
+      var ann = spans[i].__annotation;
+      if (ann) return popupSpanTooltip(ann, e);
     }
   }