Remove old/unnecessary reboot bug fix

[patrickhlauke]

From initial testing, this bug ("transparent button background results in a loss of the default button focus styles") doesn't seem to manifest itself anywhere in Bootstrap (since we don't just set transparent background anywhere on buttons, and when we do set explicit button styles in the more specific stylings, we already do create a custom :focus style anyway)

Incidentally, this forced definition of :focus is biting us in the rear because it essentially circumvents the browser logic that only applies focus outline based on the browser's heuristic (i.e. not applying the button outline when clicked with a mouse). this forces the focus style to always show even after a mouse click, which is causing is hassle that we then need to work around - see #32630

[patrickhlauke]

Testing with Firefox (and IE, though of course v5 isn't aimed at it, but the principle is the same for v4.x which does include it) on https://deploy-preview-32631--twbs-bootstrap.netlify.app/docs/5.0/content/reboot/#forms and https://deploy-preview-32631--twbs-bootstrap.netlify.app/docs/5.0/components/buttons/#outline-buttons and various other places, I see no issues with "transparent button background results in a loss of the default button focus styles" per the comment. It may well be that we've been cargo-culting this fix unnecessarily.

[patrickhlauke]

just want to wait/check with others like @mdo who may know of the original reason that patch was added in the first place... (or i could go on an archeological expedition to find the commit where this was first added)

[septatrix]

In the future it should be possible to use :focus-visible to only show focus styles when the browser determines they are necessary however support is currently not perfect

[XhmikosR]

Not sure if this should be backported, though.

[patrickhlauke]

i'd say it's worth backporting as it shouldn't actually affect anything in v4. and other PRs that i think are worth backporting as well (e.g. #32630 (review)) rely on this PR to be backported if they're also to be backported

[patrickhlauke]

seems that while removing this fixes the unsightly behaviour in firefox (for when buttons are used as tab riders), something still trips up chromium making it show its default outline even on mouse press. this may need a further fix. adding

button:focus:not(:focus-visible) {
  outline: 0;
}

to reboot seems to work (and as it seems to only still be a problem in chromium, and recent chromium is one of the few browsers that supports :focus-visible, this seems like a fair enough addition...thoughts?)

[XhmikosR]

@mdo @MartijnCuppens please confirm if this can safely be backported or not.