When ClutterImage got merged into StImageContent with commit 64d5d01360
("st: Merge ClutterImage with ImageContent"), a small error slipped in when
copying over the update_image_size() function.
Within ClutterImage, update_image_size() served a simple purpose: To call
clutter_content_invalidate_size() in case the texture size would change.
To check that, ClutterImage was storing the existing texture size in the
priv->width and priv->height fields.
When merging into StImageContent, those priv->width/height fields happened
to overlap with priv->width/height of StImageContent, but there they are
storing the preferred size, rather than the texture size. Copying
update_image_size() as-is into StImageContent meant we now set the
preferred size to the texture size, subtly breaking resource-scaled StIcons
(where texture size is a multiple of the preferred size).
The breakage isn't very apparent because StIcons manually set the size
of their internal ClutterActor child to the requested icon-size via
clutter_actor_set_size() in st_texture_cache_load_gicon(), and that makes
the child actor report the correct size in get_preferred_width/height(). The
child actor gets allocated via clutter_actor_allocate_align_fill() from
ClutterBinLayouts vfunc_allocate(), and clutter_actor_allocate_align_fill()
honors both the available size from the parent and the CONTENT_SIZE
request-mode on the child. So when the StIcon has a larger allocation (aka
extra available size), clutter_actor_allocate_align_fill() will fill that
available space with the (too large) content size.
To reproduce the issue, on a resource-scaled monitor, simply create an
StIcon with a fixed width and height that's larger than the icon-size. The
icon will be painted too large and blurry.
Fix this fallout by avoiding to touch the priv->width/height properties on
StImageContent, and instead simply comparing old texture size to new texture
size, we have both textures around after all!
Fixes: 64d5d01360 ("st: Merge ClutterImage with ImageContent")
Closes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/8287
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3680>
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