stretch to fill all space if possible, center if no meaningful way to stretch
snap to left or top side, leaving space on right or bottom
snap to right or bottom side, leaving space on left or top
center natural width of widget inside the allocation
align the widget according to the baseline. Since 3.10.
Controls how a widget deals with extra space in a single (x or y) dimension.
Alignment only matters if the widget receives a “too large” allocation, for example if you packed the widget with the expand flag inside a gtk.Box, then the widget might get extra space. If you have for example a 16x16 icon inside a 32x32 space, the icon could be scaled and stretched, it could be centered, or it could be positioned to one side of the space.
Note that in horizontal context GTK_ALIGN_START and GTK_ALIGN_END are interpreted relative to text direction.
GTK_ALIGN_BASELINE support for it is optional for containers and widgets, and it is only supported for vertical alignment. When its not supported by a child or a container it is treated as GTK_ALIGN_FILL.