Try pasting this in the IDE:

class A{
    class B{
        class C{
}}}

and then collapse "class C". You'll see a bad GUI artifact for the last fold "mark". The problem being it still draws an
extra horizontal line to connect to "something" only that it doesn't have what to.

The same problem happens with Javascript for example:

function a(){
    
  function b(){
  }}

after you collapse "b()".

The bug seems to be in CodeFoldingSideBar as it always draws the two horizontal lines if getInnerLevel() > 0. Looks to
me like PaintInfo doesn't have enough information to cover this situation (folded, PAINT_MARK,InnerLevel>0 with nothing
following to "connect" visually to).

My patch against CodeFoldingSideBar involves separating SINGLE_PAINT_MARK from PAINT_MARK on collapsed folds just to
indicate this situation. Basically if a collapsed fold has all the parents ending on the same row (that is, no
PAINT_END_MARK or PAINT_LINE will follow on the next line) it must be called SINGLE_PAINT_MARK, otherwise PAINT_MARK .

Sorry for the inlined patch, I used diff, not hg to produce it.

@@ -301,9 +301,25 @@
                     viewRect = viewShape.getBounds();
                     y = viewRect.y + viewRect.height;
                     boolean isSingleLineFold = startViewIndex == endViewIndex;
+                    //emi: the check bellow seems to be redundant, fold.isCollaped determines isSingleLineFold to be true
                     if (fold.isCollapsed() || isSingleLineFold){
+                        boolean foldedSinglePaint = fold.isCollapsed(); //parents end here too ?
+                        if(foldedSinglePaint){
+                            //see if the parents ends on the same line and mark it as SINGLE_PAINT_MARK 
+                            int off = fold.getEndOffset();
+                            int rowEnd = javax.swing.text.Utilities.getRowEnd(component, off);
+                            Fold parent = fold.getParent();
+                            while(parent!=null && !FoldUtilities.isRootFold(parent)){
+                                if(rowEnd!=javax.swing.text.Utilities.getRowEnd(component, parent.getEndOffset())){
+                                    foldedSinglePaint = false;
+                                    break;
+                                }
+                                parent = parent.getParent();
+                            }
+                        }
+                        boolean singleMarkKind = fold.isCollapsed() ? foldedSinglePaint : isSingleLineFold;
                         map.put(new Integer(viewRect.y), 
-                            new CodeFoldingSideBar.PaintInfo((isSingleLineFold?SINGLE_PAINT_MARK:PAINT_MARK), level,
viewRect.y, viewRect.height, fold.isCollapsed()));
+                            new CodeFoldingSideBar.PaintInfo((singleMarkKind?SINGLE_PAINT_MARK:PAINT_MARK), level,
viewRect.y, viewRect.height, fold.isCollapsed()));
                         return;
                     }
 
@@ -534,7 +550,7 @@
                     }
                     if (paintInfo.getInnerLevel() > 0){ //[PENDING]
                         g.drawLine(lineX, y, lineX, markY);
-                        g.drawLine(lineX, markY + markSize, lineX, y + height);
+                        if (paintOperation != CodeFoldingSideBar.SINGLE_PAINT_MARK) g.drawLine(lineX, markY + markSize,
lineX, y + height);
                     }
                     visibleMarks.add(new Mark(markX, markY, markSize, isFolded));
                 } else if (paintOperation == PAINT_LINE){