[REG 5.10.0->5.11.3] Widgets break out and overlap Tree View columns.

In a QTreeWidget (or View), in previous versions of QT the boundaries of columns were respected and no widget was capable of breaking through a column. We recently discovered all the formatting was wrong in our tree views on upgrading from 5.6.3 to 5.12.3. This was discovered in a larger project, but has been narrowed down to a small test project uploaded here where the issue can be reproduced.

UPDATE - Have narrowed this down as having happened 5.10.0 -> 5.11.3 and updated subject accordingly.

Steps:

1. Create a QTreeWidget
2. Populate with a few columns
3. Have a variety of widgets in columns (QComboBox with a long text is a particularly bad offender for testing this bug)
4. Start dragging column headers to resize column widths.

Expected:

• Columns resize and the contents respect boundaries.

Actual:

• Column contents draw over the boundaries of tree view cells.