How to fix Flutter dropdown button overflow issue?

Though I have flagged the question as possible duplicate, a partial solution not mentioned in the other question is to use the isExpanded property for DropDownButton.

              child: new DropdownButton<String>(
                isExpanded: true,
                ...

In most cases, in addition to the expanded, it is also good to make it ellipsized.. steps 1 and 2. If it's not ellipsized it will make it wrap to next line and if the component doesn't support multiples lines it will truncated the text.

DropdownButton(
    isExpanded: true, //Step 1
    items: [
        new DropdownMenuItem(
            child: Text("Long text that overflow the size.. wrapped or ellipsized", 
            overflow: TextOverflow.ellipsis),  //Step 2
        ),
    ],
    onChanged: (val) { }
)