wip scroll: Keep short threads pinned at bottom, not past bottom; TODO test

This is NFC as to the real message list, because so far the bottom
sliver there always has height 0.

This is a step toward letting us move part of the message list into
the bottom sliver, because it fixes a bug that would otherwise create
in the case of a thread that fits entirely on the screen.

Author: gnprice

lib/widgets/scrolling.dart

@@ -273,9 +273,42 @@ class MessageListScrollPosition extends ScrollPositionWithSingleContext {
   /// 0.0 and 60.0, or -10.0 and 10.0, or -40.0 and 0.0, or other values,
   /// depending on the value of [Viewport.anchor].
   bool applyContentDimensionsRaw(double wholeMinScrollExtent, double wholeMaxScrollExtent) {
-    // This makes the simplifying assumption that `anchor` is 1.0.
-    final effectiveMin = math.min(0.0, wholeMinScrollExtent + viewportDimension);
+    // The origin point of these scroll coordinates, scroll extent 0.0,
+    // is that the boundary between slivers is the bottom edge of the viewport.
+    // (That's expressed by setting `anchor` to 1.0, consulted in
+    // `_attemptLayout` below.)
+
+    // The farthest the list can scroll down (moving the content up)
+    // is to the point where the bottom end of the list
+    // touches the bottom edge of the viewport.
     final effectiveMax = wholeMaxScrollExtent;
+
+    // The farthest the list can scroll up (moving the content down)
+    // is either:
+    //   * the same as the farthest it can scroll down,
+    //   * or the point where the top end of the list
+    //     touches the top edge of the viewport,
+    // whichever is farther up.
+    final effectiveMin = math.min(effectiveMax,
+      wholeMinScrollExtent + viewportDimension);
+
+    // The first point comes into effect when the list is short,
+    // so the whole thing fits into the viewport.  In that case,
+    // the only scroll position allowed is with the bottom end of the list
+    // at the bottom edge of the viewport.
+
+    // The upstream answer (with no `applyContentDimensionsRaw`) would
+    // effectively say:
+    //   final effectiveMin = math.min(0.0,
+    //     wholeMinScrollExtent + viewportDimension);
+    //
+    // In other words, the farthest the list can scroll up might be farther up
+    // than the answer here: it could always scroll up to 0.0, meaning that the
+    // boundary between slivers is at the bottom edge of the viewport.
+    // Whenever the top sliver is shorter than the viewport (and the bottom
+    // sliver isn't empty), this would mean one can scroll up past
+    // the top of the list, even though that scrolls other content offscreen.
+
     return applyContentDimensions(effectiveMin, effectiveMax);
   }
 }