InputDialog: set class default for keyboard_visible to false (#12994)

frontend/ui/widget/inputdialog.lua

@@ -614,7 +614,6 @@ function InputDialog:onShowKeyboard(ignore_first_hold_release)
     end
     -- NOTE: There's no VirtualKeyboard widget instantiated at all when readonly,
     --       and our input widget handles that itself, so we don't need any guards here.
-    --       (In which case, isKeyboardVisible will return `nil`, same as if we had a VK instantiated but *never* shown).
     self._input_widget:onShowKeyboard(ignore_first_hold_release)
     -- There's a bit of a chicken or egg issue in init where we would like to check the actual keyboard's visibility state,
     -- but the widget might not exist or be shown yet, so we'll just have to keep this in sync...

frontend/ui/widget/inputtext.lua

@@ -777,6 +777,8 @@ function InputText:isKeyboardVisible()
     if self.keyboard then
         return self.keyboard:isVisible()
     end
+    -- NOTE: Never return `nil`, to avoid inheritance issues in (Multi)InputDialog's keyboard_visible flag.
+    return false
 end
 
 function InputText:lockKeyboard(toggle)

frontend/ui/widget/virtualkeyboard.lua

@@ -781,7 +781,7 @@ end
 
 local VirtualKeyboard = FocusManager:extend{
     name = "VirtualKeyboard",
-    visible = nil,
+    visible = false,
     lock_visibility = false,
     covers_footer = true,
     modal = true,