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[ListItemDragHandle] Bugfix: Auto-scroll upwards not working due to rounding real values to integers

M. Sadık Uğursoyrequested to merge
ugursoy/kirigami:drag-handle-floating-point-precision into master

Description

Brief: Auto-scroll upwards works inconsistently due to a rounding down operation caused by mismatched variable types.

In onPositionChanged callback, the following assignments cause the bug because listItem.y is a real value and internal.listItemLastY is defined as integer.

@@210 internal.draggingUp = listItem.y < internal.listItemLastY
@@211 internal.listItemLastY = listItem.y;

@@216 scrollTimer.running = (listView.contentHeight > listView.height)
                && ((listItem.y === 0 && !listView.atYBeginning)
                    || (listItem.y === mouseArea.drag.maximumY && !listView.atYEnd));

Case

  • In order to detect internal.draggingUp, current listItem.y and previous one is compared
  • Assume current listItem.y is close to 0 but not 0 (0.02)
  • It will be stored as 0
  • In the next callback to the same function, the new listItem.y will be zero if user keep dragging upwards
  • But instead of 0 < 0.02 comparison, the code will compare 0 < 0 and return false
  • As a result, it will not auto-scroll top

Solution

  • Make listItemLastY a real type
  • Even though mouseDownY and startY are irrelevant in this bug, they should be real type too because the values they are assigned to are real values and may cause bugs for other behaviors

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