ScrollFinisher: reset targetPos after reaching the target

Otherwise the old targetPos stands in the way of correct scrolling if
the read marker is out of the viewport in the next room the user
switched to.

client/qml/ScrollFinisher.qml

@@ -114,8 +114,9 @@ Timer {
             // repeat positioning; otherwise, position the canvas exactly
             // where it should be
             if (targetPos) {
-                // this.contentY is readonly
+                // NB: this.contentY is a readonly alias
                 parent.contentY = targetPos
+                targetPos = undefined
                 return true
             }
             parent.positionViewAtIndex(targetIndex, positionMode)