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This ist actually multiple issues that may come from a single point of failure. On OSX 10.10.3 with Netbeans already started, when I connect a second display and the windows are rearranged / placed by OSX, Netbeans is assigned to the second display. OSX seems to memorize the last position and screen affinity somehow. However, the Netbeans window ist stuck in the lower half of the screen. You can drag it to the top an it instantly drops back into the lower half. The only workaround I found was to maximize (fullscreen, green button) Netbeans and then minimize it again. This seems to reset the bounds. No other OSX programs show this behaviour, so maybe it arises from the Java Window Manager. Secondly, sometimes all internal panels and windows inside Netbeans are drastically minimized, so that it is only possible to reset the windows configuration. If the bug cannot be located, I would appreciate a "save current window configuration" button.
Based on the description of the first issue this sounds like a duplicate. See the attached video on that bug. *** This bug has been marked as a duplicate of bug 250825 ***