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Resizing the browser with the top right "maximize" button of the window does not resize the left pane.
If you click the maximize button (and your browser should shrink in height, otherwise, do it yourself while dragging the bottom border of the window) press the "maximize" window button again, the left pane didn't resize now.
Also when dragging the bottom border it does resize, but it's not always the correct height.
Tested in Chrome 43 and IE 11 on Windows 7 with SmarterMail 14.0.5647