Angular scope variable update not reflected in UI

We are working on an HTML page which makes use of a Bootstrap tooltip on a certain <span> tag. For those who have not heard of tooltip, it is a popup of sorts which appears when hovering over the element to which it is attached. Here is a screenshot showing the <span> in question, and what happens on hover:

The premise behind adding the tooltip was that in the event that we truncate the text, the tooltip would provide an option for viewing the entire text.

However, we would now like to condtionally show the tooltip only when there is no ellipsis in the text. We defined the tooltip-enable property in the <span>:

<span uib-tooltip="{{someName}}" tooltip-placement="right" tooltip-enable="{{showToolTip}}">{{someNameShortened}}</span>

The key thing here is tooltip-enable="{{showToolTip}}", which binds the property to a scoped variable in the controller for this page. And here is the relevant (and abbreviated) controller code:

mainApp.controller('repoListController',['$scope', '$rootScope', ...,
function($scope,$rootScope, ...) {

    $scope.showToolTip = false;

    var repositoryList= function(){
        repositoryService.getRepositoryList(function(data) {
            var repoList = data;                
            repoList.shortenedDisplayName = repositoryService.getShortRepoName(repoList.repoName, DISPLAY_NAME_MAX_LENGTH);
            // if the repository's name be sufficiently large (i.e. it has an ellipsis)
            // then show the tooltip.  Otherwise, the default value is false (see above)
            if (repoList.repoName.length > DISPLAY_NAME_MAX_LENGTH) {
                $scope.showTooltip = true;
            }
        });
    }

    repositoryList();
}]);

Based on the research I have done, the common solution for why a change to a scoped variable is not reflected in the UI is to run $scope.$apply(), or some variation on this. Running apply(), as I understand it, will tell Angular JS to do a digest cycle, which will propagate changes in the scope to the UI. However, trying to do an apply() from the code which toggles showToolTip resulted in errory. I inspected the value of $scope.$root.$$phase while running the code which updates the showToolTip variable, and the phase was digest.

So now I am at a loss to explain this. If the code is already in a digest, then why would changes not be reflected in the UI? Also, if the code is already in digest, then how could I force Angular to sync the changes to the UI?