feat: correct z-index level for floating always rendered panel

[mathuo]

[codesandbox-ci[bot]]

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Sandbox	Source
customheader-dockview	Configuration
dockview-app	Configuration
editor-gridview	Configuration
events-dockview	Configuration
externaldnd-dockview	Configuration
fullwidthtab-dockview	Configuration
iframe-dockview	Configuration
keyboard-dockview	Configuration
lockedgroup-dockview	Configuration
nativeapp-dockview	Configuration
nested-dockview	Configuration
rendering-dockview	Configuration