During a flyTo, the zoom level changes out of its boundaries.

mapbox-gl-js version: 0.49

browser: Chrome 69.0.3497.100

Steps to Trigger Behavior

Add this test case to the unit test camera.test.js:

        t.test('Zoom out with animation without over out-zooming', (t) => {
            const pos = { lng: 0, lat: 0 };
            const startZoom = 19;
            const endZoom = 18;
            const camera = createCamera({zoom: startZoom, center: pos});
            const stub = t.stub(browser, 'now');

            stub.callsFake(() => 0);
            camera.flyTo({ zoom: endZoom, center: pos, duration: 1000 });

            stub.callsFake(() => 999);
            camera.simulateFrame();
            t.ok(camera.getZoom() >= endZoom && camera.getZoom() < startZoom);
            t.end();
        });

Almost at the end of the flight, the zoom level is equal to 17.4, much lower than 18.

Expected Behavior

The zoom level does not go below the final zoom level during the flight.

Actual Behavior

The zoom level goes below the final zoom level during the flight, but it gets corrected instantly at the end of the flight. This produces a strange effect from the user

Note that changing the initial and final zoom level (e.g. 20 -> 19) may not show the problem.

I tried to look a the code the compute the easing function parameter, but without any luck (the computations are beyond my knowledge :-) )