RangeSelector: rename m_mousePos to m_mouseDownPos
RangeSelector: draw cursor position on the bar
RangeSelector: bug fix: reset m_mouseDown to false on mouse release
[refactor] RangeSelector: extract edgeToMove() from mouseDown()
RangeSelector: simplify the logic of edgeToMove()
RangeSelector: change the cursor to CrossCursor
RangeSelector: draw arrow of expected change of value
RangeSelector: hide arrow when dragging