Add a min-height to stop the back to top link collapsing

[steventux]

Addresses #786

A good test page is https://government-frontend-pr-787.herokuapp.com/drug-device-alerts/drug-alert-amoxicillin-sodium-500mg-powder-for-solution-for-injection-additional-reports-of-injection-site-reactions-in-paediatrics-and-adults?cachebust=1519124601

Here's a video of it misbehaving:

The elements below the back to top link can briefly
appear above the link as it transitions from an in-body
element to a fixed position, adding a minimum height stops the
element collapsing and causing this flickering element
reordering.

Thanks to @andysellick and @fofr for reporting and explaining this.

---

Component guide for this PR:
https://government-frontend-pr-787.herokuapp.com/component-guide

[fofr]

Looks like this isn't currently working as expected.

[steventux]

@fofr can you elaborate? I've tested this in several browsers now, the original commit wasn't successful as it was a bit simplistic but since 64b5e94 I can't replicate the same jumpy behaviour. I've now tested on Safari 8, Chrome 64 and IE9. I've been able to replicate it (intermittently) on Firefox 58 if I use arrow keys but otherwise scrolling no longer produces a noticeable jump. Perhaps I'm used to working around the issue? Any info on how to replicate would be handy.

Would you say this latest commit is an improvement at all? If so I'd be inclined to accept this short term fix until we have time to dive a bit deeper into the problem.

[vanitabarrett]

@steventux @fofr Possibly a caching issue - I thought I could replicate on https://government-frontend-pr-787.herokuapp.com/drug-device-alerts/drug-alert-amoxicillin-sodium-500mg-powder-for-solution-for-injection-additional-reports-of-injection-site-reactions-in-paediatrics-and-adults?cachebust=1519124601 on Chrome too, but then refreshed and the problem seems to be resolved with the latest commit.

[steventux]

@vanitabarrett would you say this improves the behaviour? I think if we've improved the behaviour this is a harmless if slightly simplistic way of improving user experience.

[vanitabarrett]

@steventux It definitely seems to improve the problem - I've just quickly checked on latest versions of Safari, Firefox and Chrome, and IE 9 on browserstack, and I can't replicate the issue any more

[steventux]

@fofr @vanitabarrett this PR could do with approval if it's deemed to at least improve the behaviour.

[vanitabarrett]

@steventux I can't seem to replicate this issue any more on the latest master build of government-frontend so I'm wondering if this fix is still needed? Testing in Chrome 64

[maxgds]

I can't replicate the behaviour using this branch, I think we should merge as it's an improvement. If we find further issues we can address them separately.

[steventux]

Very strange, I know some small stylistic tweaks were applied to the feedback component, perhaps the additional margin before the feedback component/after the content has negated the space where the two elements appeared to switch place. I'll close this then, thanks 👍