Fix stacking context of floating toolbar

[loichuder]

Before	After

[axelboc]

You also need to create a stacking context, perhaps on canvasWrapper so the z-index of the floating toolbar does not "leak" to the document's stacking context.

You can replicate this (potential) issue by giving a higher z-index value to the floating toolbar, opening the overflow menu of the main toolbar, and resizing the window so the overflow menu appears under the reset-zoom button.

By setting z-index: 0 on .canvasWrapper (and thus creating a new stacking context, since it already has position: relative), the risk of a future stacking problem disappears.