Bug 284138 [CSS] ETabItem wrong background color behind X
If the close box is not shown (but space is allocated, e.g. editors) then the a gray background is drawn in it's place. It's the wrong color background though, it should be now the unselected tab background color.
bundles/org.eclipse.e4.ui.widgets/src/org/eclipse/e4/ui/widgets/ETabItem.java
int xx = parent.borderLeft;
	int yy = parent.borderTop + height + 1;
	int ww = size.x - parent.borderLeft - parent.borderRight;
	int hh = parent.highlight_header - 1;
	int[] shape = new int[] {xx,yy, xx+ww,yy, xx+ww,yy+hh, xx,yy+hh};
	if (parent.selectionGradientColors != null && !parent.selectionGradientVertical) {
		parent.drawBackground(gc, shape, true);
	} else {
		gc.setBackground(parent.selectionBackground);
		gc.fillRectangle(xx, yy, ww, hh);
	}
	
	
	
	if (!showing){
		int x1 = Math.max(0, parent.borderLeft - 1);
		int y1 = y + height;
		int x2 = size.x - parent.borderRight;
		gc.setForeground(getETabParent().tabKeylineColor);
		gc.drawLine(x1, y1, x2, y1);
		return;
	}
		
	
	shape = null;
	
	int[] left = ETabFolder.E_TOP_LEFT_CORNER;
	int[] right = ETabFolder.E_TOP_RIGHT_CORNER;
	shape = new int[left.length+right.length+8];