Accessibility Issue, Keyboard Navigation not possible

[Nimo-19]

During our Update from Plone 5.2 to Plone 6 we noticed, that the Navigation via Keyboard in the Mosaic Editor does not work anymore.

It is not possible to activate the Toolbar-Buttons via Enter. Some Event Handler seems to prevent it.
Even if you manage to open the 'Properties' Modal somehow, it is not possible to move to a different tab than the 'Default' tab, again some eventhandler seems to prevent the enter key from working. (On a normal Plone edit view this works as expected).

In addition it seems not possible to reach the Edit button (e.g. on a Content listing tile) of a tile via the keyboard.

I tried all the above with a fresh plone classic 6.0.8 and plone.app.mosaic 3.1.5

[Nimo-19]

Accessing the edit button ( e.g. on the Content listing tile) is somehow prevented by the keydown listner in mosaic.layout.js#L353
Don't know what this is supposed to do, but it seems prevent the edit button from being accessed.
Seems to me this should be somehow for new tiles. So eventueally query for .closest('.mosaic-tile:not(:focus-within)') in mosaic.layout.js#L359
Seems to work for me. Not sure if it breaks something different.

Additionally Sometimes the control buttons aren't event visible. Pure focus via tab does not 'select' the tile it seems.

Maybe make the buttons visible on .mosaic-tile:focus-within instead of .mosaic-selected-tile (or in addition)

[Nimo-19]

The issue that the toolbar buttons can not be activated via keyboar can be fixed by switching from mousedown to click event in /mosaic.toolbar.js#L399