Question to workspaces preview in overview

In overview mode, you have these little preview thumbnails for the workspaces under the search bar. A blue frame inicates the cureent workspace. Sometimes, this blue frame has little blue squares in its corners, sometimes not. Are theses squares a feature? Are they indicating something? Or is this a bug?
I am in Solus with GNOME 44.3

Bildschirmfoto vom 2023-10-13 12-02-50

Bildschirmfoto vom 2023-10-13 12-02-29

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