Bug Report

Ionic version:

[x] 4.x

Current behavior:
The background of ion-tab-button elements remains unchanged when they receive focus, even though the --background-focused CCS4 variable for ion-tab-button elements is set.

Expected behavior:
I expected the ion-tab-button elements' background to get the background color I set in the --background-focused variable.
This is supposed to happen if I understand the docs correctly, see
https://beta.ionicframework.com/docs/api/tab-button

Steps to reproduce:

1. Create an Ionic tabs starter.
2. In the tabs.page.scss file paste:

ion-tab-bar {
  --background: grey;
}
ion-tab-button {
  --color: blue;
  --color-selected: red;
  --background-focused: orange;
}

1. Run ionic serve, and preview in Firefox (don't see it working in other browsers either).
2. Move along the tab buttons by using the physical "Tab" key on the keyboard.
3. Observe that when a tab button receives focus its background remains unchanged.

Related code:

Other information:

Ionic info:

Ionic:

   ionic (Ionic CLI)             : 4.5.0
   Ionic Framework               : @ionic/angular 4.0.0-rc.0
   @angular-devkit/build-angular : 0.8.7
   @angular-devkit/schematics    : 7.1.2
   @angular/cli                  : 6.2.8
   @ionic/angular-toolkit        : 1.1.0

System:

   NodeJS : v8.11.2 (C:\Program Files\nodejs\node.exe)
   npm    : 5.6.0
   OS     : Windows 10