Very little feedback is given to the user when they attempt to drag items which cannot be moved. These items are currently somewhat faded, and the regular "pointer"-cursor is displayed when hovering. But no feedback is given when a drag is attempted. This is especially a problem with the tab-bar overflow drop-down menu, where items can be placed either to the left or right of said button. With this in mind, it might be a good idea to use either the "no-drop"-cursor or the "not allowed"-cursor when hovering immovable toolbar/panel items. To view the cursors see: https://developer.mozilla.org/en-US/docs/Web/CSS/cursor. This may depend on bug 275173 (On Windows and OS X, "no-drop" is the same as "not-allowed").
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