Adds a child at fixed coordinates in one of the text widget's windows.
The window must have nonzero size (see set_border_window_size). Note that the child coordinates are given relative to scrolling. When placing a child in WIDGET, scrolling is irrelevant, the child floats above all scrollable areas. But when placing a child in one of the scrollable windows (border windows or text window) it will move with the scrolling as needed.
| this |
a TextView |
| child |
a Widget |
| which_window |
which window the child should appear in |
| xpos |
X position of child in window coordinates |
| ypos |
Y position of child in window coordinates |