KivyMD, MDFloatingActionButton: correct placement - kivy

Question

I've been trying to use floating action buttons alongside FileChooser:
import kivy
kivy.require("2.1.0")
from kivy.utils import platform
from kivy.lang import Builder
from kivy.uix.floatlayout import FloatLayout
from kivymd.app import MDApp
from kivymd.uix.tab import MDTabsBase
from kivymd.icon_definitions import md_icons
import os
USERPATH = os.path.expanduser("~")
if platform == "android":
from android.storage import primary_external_storage_path
from android.storage import secondary_external_storage_path
from android.permissions import request_permissions, Permission
USERPATH = primary_external_storage_path()
request_permissions(
[Permission.WRITE_EXTERNAL_STORAGE, Permission.READ_EXTERNAL_STORAGE]
)
KV = '''
#:kivy 2.1.0
#:set dark_gray (.5, .5, .5, 1)
MDBoxLayout:
orientation: "vertical"
MDTabs:
id: ps_tabs
TabList:
id: ps_tab_list
icon: "folder"
FileChooserListView:
id: ps_filechooser
canvas.before:
Color:
rgba: dark_gray
Rectangle:
size: self.size
pos: self.pos
MDFloatingActionButton:
id: fc_playdir
icon: "folder"
pos_hint: {"center_x": .4, "center_y": .5}
MDFloatingActionButton:
id: fc_playfile
icon: "file"
pos_hint: {"center_x": .6, "center_y": .5}
TabDetails:
id: ps_tab_details
icon: "book"
MDScrollView:
'''
class TabList(FloatLayout, MDTabsBase):
"""The engaged power supplies tab."""
class TabDetails(FloatLayout, MDTabsBase):
"""The engaged power supply details tab."""
class Ron(MDApp):
def build(self):
return Builder.load_string(KV)
def on_start(self):
self.theme_cls.primary_palette = "Gray"
self.theme_cls.material_style = "M3"
if platform != "android":
self.root.ids.ps_tabs.lock_swiping = True
self.root.ids.ps_filechooser.rootpath = USERPATH
if __name__ == "__main__":
Ron().run()
Mouse clicks, as well as taps, shoot right through the buttons and into the FileChooser, no matter what I do. I tried placing the buttons into their own box layout, tried moving them up and down the widget tree. Nothing helps, while the buttons remain visible. Besides, on visiting the second tab, one can see two thick black button footprints. Something's very wrong and I can't think of anything else to try.