XPLM_API void XPLMSetWindowGravity( XPLMWindowID inWindowID, float inLeftGravity, float inTopGravity, float inRightGravity, float inBottomGravity);
A window’s “gravity” controls how the window shifts as the whole X-Plane window resizes. A gravity of 1 means the window maintains its positioning relative to the right or top edges, 0 the left/bottom, and 0.5 keeps it centered.
Default gravity is (0, 1, 0, 1), meaning your window will maintain its position relative to the top left and will not change size as its containing window grows.
If you wanted, say, a window that sticks to the top of the screen (with a constant height), but which grows to take the full width of the window, you would pass (0, 1, 1, 1). Because your left and right edges would maintain their positioning relative to their respective edges of the screen, the whole width of your window would change with the X-Plane window.
Only applies to modern windows. (Windows created using the deprecated XPLMCreateWindow(), or windows compiled against a pre-XPLM300 version of the SDK will simply get the default gravity.)