layer contextmenu does not get focused when opened with Shift+F10

[AliyanH]

Layer contextmenu does not function as it used to. Tabbing to the layer and opening the layer contextmenu using shift+F10 does not move focus to the layer contextmenu. Might have been caused after the leaflet 1.9.3 upgrade. Need to fix and add tests.

After Leaflet 1.9.3 upgrade, opening the layercontexmenu would open the browser's default contextmenu, this commit tried to fix the changes but in doing so we lost the ability to focus on the layer contextmenu.

[yhy0217]

Noticed that this bug happens because layer context menu does not really go into the if statement inside _show function (shown below), which sets where the focus should fall onto when menu is accessed and opened by using Shift + F10:

It is because the button attribute is only available for instances of class PointerEvent.

And if access map and layer context menu in browser using keyboard/mouse, and print the event object in the console, the e.originalEvent for the map context menu is an instance of class PointerEvent:

while the e.originalEvent for the layer context menu is not:

but the two event objects both have type "contextmenu".