Not hiding the toolbar while the page is loading is a conscious design to prevent scenarios on which the dynamic toolbar is hidden and then cannot be get back. This scenario could be improved by bug 1914119.
Bug 1915714 Comment 1 Edit History
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Not hiding the toolbar while the page is loading is a conscious decision to prevent scenarios in which the dynamic toolbar is hidden and then cannot be shown back again when the page fully loads. This scenario could be improved by bug 1914119.