Unexpected resize glitches in OpenGL

Originally, I resize my window so that the video contains black borders on the left and on the right. When I resize the window to increase its height (from the bottom), the black borders are reduced, as expected:

 +--------------+          +--------------+
 |    xxxxxx    |          |  xxxxxxxxxx  |
 |    xxxxxx    |   --->   |  xxxxxxxxxx  |
 |    xxxxxx    |          |  xxxxxxxxxx  |
 +--------------+          |  xxxxxxxxxx  |
                           +--------------+

So far, so good.

But during the resize, the top of the video does not match the top of the window for a fraction of seconds, producing an unpleasant blinking with a black border on the top.

 +--------------+
 |              | <--- unexpected black border for a fraction of seconds
 |  xxxxxxxxxx  |
 |  xxxxxxxxxx  |
 |  xxxxxxxxxx  |
 +--------------+

The issue happens only with the OpenGL vout -Vgl (tested on Linux/X11). It does not happen with -Vlibplacebo or -Vxcb_render. EDIT: in fact, it happens with libplacebo too: #26734 (comment 314599)

Using Qt or -Idummy makes no difference.