KQuickStyleItem manages its implicit size internally. Overriding it on the QML side makes this non-deterministic which assignment wins and might cause unexpected re-evaluation of the size causing it to change.

Moreover, the CheckIndicator implicit size is based on sizeFromContents for CT_CheckBox, which in case of Breeze (but not the Qt built-in styles) already contains some extra padding on the side between checkbox and label. Instead of using that for layouting our full checkbox (indicator + label), do what qqc1 did and use PM_CheckBoxLabelSpacing for spacing and PM_IndicatorWidth as size hint for the indicator.

Also while at it for radio buttons use the appropriate (PM_RadioButtonLabelSpacing and PM_ExclusiveIndicatorWidth) hints.

This makes QQC2 CheckBox and RadioButton layouted pixel-perfect to their QWidget counterparts and also fixes it randomly changing size hints as you switch between pages as demonstrated by the bug report.

BUG: 418447

The only downside might be that using a CheckIndicator standalone now has a quite small size (just the checkbox you see basically) rather than a "nice delegate like height". But then I don't think this control is being used anywhere, and is probably centered vertically anway. When used in conjunction with ItemDelegate (checkable item delegate) I believe the entire delegate becomes clickable to toggle the CheckBox so it getting smaller should have no effect.

Also, using those invokables causes slight layout problems when switching between styles at runtime as they cannot be notified to have changed.

Before Breeze

Before Oxygen:

Before Windows:

(I noticed that we use an incorrect palette somewhere since those checkboxes and comboboxes are meant to be white in Windows style but that's an issue unrelated to this patch)