The yFiles library uses Java's font rendering mechanism to scale the text of node labels according to the current zoom level of the view. This rendering mechanism, however, yields non-linear results regarding the width of text, since both text spacing and glyphs can be different at subsequent zoom levels.
As a consequence, it is possible that node labels exceed the width of their nodes when the zoom level changes (both increases or decreases).

Zoom level 91%

Original size

Zoom level 110%

Zoom level 121%

Possible Solutions

A possible workaround that better keeps the label text inside its node, relies on using fractional metrics for the rendering. Fractional metrics is supported by Java by means of a rendering hint set on the Graphics2D context. The following code snippet shows how the rendering hint can be set using a customized DefaultGraph2DRenderer for a given view.

// 'view' is of type y.view.Graph2DView.

view.setGraph2DRenderer(new DefaultGraph2DRenderer() {
  protected void paint(Graphics2D gfx, NodeRealizer nr) {
    gfx.setRenderingHint(RenderingHints.KEY_FRACTIONALMETRICS, RenderingHints.VALUE_FRACTIONALMETRICS_ON);
    super.paint(gfx, nr);
  }
});

As of yFiles version 2.6.1, using fractional metrics for rendering can be conveniently specified by means of the setRenderingHints method as presented in the following code snippet, for example.

// 'view' is of type y.view.Graph2DView.

RenderingHints rh = view.getRenderingHints();
rh.put(RenderingHints.KEY_FRACTIONALMETRICS, RenderingHints.VALUE_FRACTIONALMETRICS_ON);
view.setRenderingHints(rh);
YLabel.setFractionMetricsForSizeCalculationEnabled(true);

Note that for consistent label rendering results, it is strongly recommended to also call the YLabel.setFractionMetricsForSizeCalculationEnabled class method appropriately.