If the player resizes the game window and then switches away from the window (e.g. Alt + Tab), the window size and position are reset to default (center of screen, default window size). If the window is maximized (using the maximize button), it retains its size when focus is lost.
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Duplicate of MC-14323, please use the search function to see if your bug has already been submitted. Currently over 57% of tickets are being resolved as duplicate.
With further testing, it appears as if this only occurs when "docking" the window (Windows 7 and up). To reproduce, drag the title bar of the window to the very left or right side of the screen until rings appear around the cursor and let go of the mouse. The window will resize to occupy half the screen horizontally and will be vertically maximized. Upon losing focus, the window will reset to however it was before it was docked. This did not occur in 1.6.2 or earlier.