Unlock the scale bar plugin and move it slightly. Then move the pointer to a different portion of the map, click and start to drag. The scale bar will jump to the mouse position and start to follow it around. This bug also applied to Marble 1.5. Reproducible: Always
This only appears to happen if you grab the scale bar at a point that is also over another info box style plugin. (I stack my my scale bar over the "routing" info box.) It is associated with a change of the mouse pointer to the "fleur", the one associated with moving windows. The behavior will continue until you move another info box or restart Marble.
And it appears to be more general; I've duplicated it with the copyright notice. If it is stacked on top of the routing info box and then moved, it will adopt the same behavior.
See also https://git.reviewboard.kde.org/r/115622/
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Git commit 9de5a28dc12031c8910b1fc1def6621e4b359fb5 by Dennis Nienhüser. Committed on 29/03/2014 at 15:42. Pushed by nienhueser into branch 'master'. Do not filter mouse press/release events. This reverts c4e0678. Please test for possible regressions, especially the one noted in bug 242414 (when using oxygen, dragging the map moves the whole window). Related: bug 242414 M +3 -3 src/lib/marble/MarbleWidgetInputHandler.cpp http://commits.kde.org/marble/9de5a28dc12031c8910b1fc1def6621e4b359fb5