Tooltip is above and visible/usable.
Tooltip is hidden by the toolbar.

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Can confirm it. For the time being you can get around this easily with CSS:
.ck.ck-balloon-panel:not(.ck-toolbar-container) {
z-index: 1001;
}
Reason this is happening in a first place is that both toolbar and balloon panels share .ck-balloon-panel class, which have z-index applied, while balloon panel occurs later in the DOM tree.
Most helpful comment
Can confirm it. For the time being you can get around this easily with CSS:
Reason this is happening in a first place is that both toolbar and balloon panels share
.ck-balloon-panelclass, which have z-index applied, while balloon panel occurs later in the DOM tree.