The SHADE_ANIMATION_TIME variable sets the duration of the animation of
the background shading that is done when showing the overview. As
explained in the code-comment, that value must be smaller than the
animation time of the overview.
Now since we're going to start animating the background color of the
panel when showing the overview and we're going to use the overviews
animation time for that, we want to make sure the shading of the
background image and the animation of the panel are kept "in sync",
otherwise the transitions would look bad.
So slightly increase the value of SHADE_ANIMATION_TIME to 240 (the
overviews animation time is 250) to make sure those happen in the same
timeframe.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1397>
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