Pretty sure that is by design. There is a coded minimum canvas size for the icon to show and the keyboard shortcut won't enable the feature if it's not met.
Bug [ ]: Focus mode and focus mode icon display is dependent on window size

Pretty sure that is by design. There is a coded minimum canvas size for the icon to show and the keyboard shortcut won't enable the feature if it's not met.
Please clarify. There is a ton of space available for the icons as the others display and there is lots of vertical space. Additionally the board displays area is adequate and focus mode permits an even larger board.
Not permitting focus mode at all when there is adequate space to do so seems to me to be a bug.
If the window is below a particular size both a game started will not be in focus mode, and the focus mode icon is not displayable..
Here are the 2 display modes:

Below is an animated gif showing how changing the zoom causes the focus more icon to disappear.