Fix tab pagination controls

[pauln]

Fixes #1063.

There were two main issues:

• fixOffsetIfNeeded() had been added into updateCanvasWidth, so it was being called on every render. This means that it ran after the nextPage action, and effectively reverted the pagination offset change because it doesn't know/care about pagination (only the selected tab)
• fixOffsetIfNeeded() was setting this.currentOffset to undefined in some situations, which then prevented pagination from working as it's used in calculations

In order to address these issues, I've removed fixOffsetIfNeeded() from updateCanvasWidth (it seems to be being called directly where needed already) and defaulted newOffset to currentOffset in fixOffsetIfNeeded(). I've done some manual testing in addition to running the automated tests, and all seems well.

[pauln]

Closing in favour of #1048, which I noticed after submitting this PR - and have since tested and confirmed that it fixes (for me, at least) both #1063 and #893, which was next on my hit list.