Redmine

Below it's a snippet from displayTabsButton() function:

    var bw = $(el).parents('div.tabs-buttons').outerWidth(true);

    if ((tabsWidth < el.width() - bw) && (lis.length === 0 || lis.first().is(':visible'))) {
      el.find('div.tabs-buttons').hide();
    } else {
      el.find('div.tabs-buttons').show().children('button.tab-left').toggleClass('disabled', numHidden == 0);
    }

Selector parents('div.tabs-buttons') from line var bw = $(el).parents('div.tabs-buttons').outerWidth(true); never finds any element because div.tabs-buttons is not a parent of div.tabs.

On jQuery 2, var gw = $(el).parents('div.tabs-buttons').outerWidth(true); is null which means 0+.
On jQuery 3, var gw = $(el).parents('div.tabs-buttons').outerWidth(true); is undefined which means NaN.

Because of this change, the condition (tabsWidth < el.width() - bw) always return false on jQuery 3 because gw is NaN and tabsButton are displayed.

The attach patch fixes this incorrect behaviour and also replaces some inline styles with class hidden.