[Trackpad gestures] swipe through history only if already at page edge

• Component: WebKitGTK
• Status: NEW ---

[Kdwk]

### Current behaviour
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When zoomed in using pinch-to-zoom and not initially at left/ right screen edge, and using two fingers to scroll horizontally to reach the page edge, swiping to the previous/ next page can be immediately triggered. 

### Problem
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Accidental triggers of swiping through history often occur. When zoomed in using pinch-to-zoom, it is difficult for the user to predict where the page edge is and stop before the page edge is reached and swiping through history is triggered. As such, users sometimes overshoot when swiping to the page edge and find themselves on the previous/ next page.

### Proposal
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Only trigger swiping through history if the user is already at the page edge, i.e. if the user is not originally at the page edge, they need to scroll horizontally to the screen edge, release, and scroll horizontally again to swipe through history. This is in line with how Safari handles this.