JComboBox does not play well with GroupLayout

Question

I ran into a strange kind of behavior when putting `JComboBox` components inside a `GroupLayout`. I've reduced the code to the below minimal example, featuring exactly one `JComboBox` layed out by `GroupLayout`. The observed behavior is as follows: - When the frame is smaller than the expanded combo box list (with dummy entries a, b, c), clicking on the little down arrow on the combo box opens it correctly. - When I resize the frame to be bigger than the expanded combo box list would be, the expanded combo box list is not opened! What I've found out already: - When the parent window of a JComboBox is too small, javax.swing.PopupFactory creates a HEAVY_WEIGHT_POPUP component. - When the parent window is big enough, a LIGHT_WEIGHT_POPUP is being created. - The behavior is definitely related to GroupLayout or some side effects of it, since any other layout managers I tried work just fine. - When using addGap() in GroupLayout the problem changes a bit, i.e. the window sizes where the JCom…

Accepted Answer

Never ever add components to the `RootPane` itself, rather add them to the `contentPane`.

```
frame.add(panel);
```

or

```
frame.setContentPane(panel);
```

A `RootPane` has control of where to put the:

- Menu bar.

- Content.

- Glass pane.

- And most important, lightweight popups (inc. `JComboBox`), dialogs, drag and drops, etc.

`RootPane` uses a special layout manager called `RootLayout`, and shouldn't be changed to `BorderLayout`.

JComboBox does not play well with GroupLayout

Solution

Problem