[trypsynth]

Fixes #150

Problem

The Kindle library list reports scrollable=false and refuses ACTION_SCROLL_FORWARD and ACTION_SCROLL_BACKWARD, and so does every container above it. Swiping stopped at the fifth book, so a bigger library was partly out of reach. A real two-finger drag does scroll it.

Change

• autoScrollAtEdge used to return when no scrollable container was found. If the pivot is inside a list or grid that refuses all scroll actions, it now calls the new dragUnscrollableList.
• New focusmanagement/DragScroller.kt:
  • Checks that the next focusable item in the swipe direction is outside the list, so the pivot is the last item on screen.
  • Drags the list with two fingers, which touch exploration passes through to the app. A one-finger drag would be explored instead.
  • The drag moves the pivot to the far edge of the list, so nothing is skipped. The fingers hold still for 100 ms with continueStroke before they lift, so the list doesn't fling.
  • After the drag, it checks whether the pivot moved. If it did, onAutoScrolled moves focus to the next item, the same as after a normal auto-scroll. If not, onAutoScrollFailed leaves the list, as before this change.
• A list that fits on screen refuses scroll actions too. So it only drags when the list's row count shows more rows, or, when there's no row count, the pivot reaches the list's edge. From an item where a drag already failed, it doesn't drag again.
• The ancestor check comes first, so screens without such a list pay only an ancestor walk.
• Only in the default granularity, vertical and logical directions, and not on TV.
• differences.md has a section under Navigation.

Testing

• DragScrollerTest: drag geometry (forward, backward, clamping, an empty list), directions, row and edge checks, and which nodes count as a list that refuses scroll actions.
• :talkback:testPhoneDebugUnitTest (542 tests) and :lintPhoneDebug pass.
• Not tested on a device. It needs checking in the Kindle library: that the injected two-finger drag scrolls it, that focus moves on to the next book, and that swiping off the last book still reaches the bottom tabs.