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
Rectangle imageBounds = image.getBounds();
		
		int maxImageWidth = x + width - xDraw - RIGHT_MARGIN;
		if (parent.showUnselectedClose && (parent.showClose || showClose)) {
			maxImageWidth -= closeRect.width + INTERNAL_SPACING;
		}
		if (imageBounds.width < maxImageWidth) {		
			int imageX = xDraw;
			int imageHeight = imageBounds.height;
			int imageY = y + getETabParent().getTextMidline() - imageHeight / 2;
			imageY += parent.onBottom ? -1 : 1;
			int imageWidth = imageBounds.width * imageHeight / imageBounds.height;
			gc.drawImage(image, 
				         imageBounds.x, imageBounds.y, imageBounds.width, imageBounds.height,
				         imageX, imageY, imageWidth, imageHeight);
			xDraw += imageWidth + INTERNAL_SPACING;
		}
	}
	
	int textWidth = x + width - xDraw - RIGHT_MARGIN;
	if (parent.showUnselectedClose && (parent.showClose || showClose)) {
		textWidth -= closeRect.width + INTERNAL_SPACING;
	}
	if (textWidth > 0) {
		Font gcFont = gc.getFont();
		gc.setFont(font == null ? parent.getFont() : font);
		if (shortenedText == null || shortenedTextWidth != textWidth) {
			shortenedText = shortenText(gc, getText(), textWidth);
			shortenedTextWidth = textWidth;
		}