sticky_header [nfc]: Expand on the header-overflows-sliver case

This case has several bugs in it.  Not coincidentally, it's tricky to
think through: there are several sub-cases and variables involved
(the growth direction, vs. the header-placement direction, vs. the
coordinate direction, ...).  And in fact my original PR revision
which fixed the cases that would affect the Zulip message list
was still conceptually confused, as evidenced by the fact that it
turned out to break other cases:
  #1316 (comment)

One step in sorting that out was the preceding commit which split
this overflows-sliver case from the alternative.  As a next step,
let's expand on the reasoning here a bit, with named variables
and comments.  In doing so, it becomes more apparent that several
points in this calculation are wrong; for this NFC commit, mark
those with TODO-comments.  We'll fix them shortly.

Author: gnprice

lib/widgets/sticky_header.dart

@@ -628,13 +628,28 @@ class _RenderSliverStickyHeaderList extends RenderSliver with RenderSliverHelper
           // The header will overflow the child sliver.
           // That makes this sliver's geometry a bit more complicated.
 
+          // This sliver's paint region consists entirely of the header.
+          final paintExtent = headerExtent;
+          headerOffset = _headerAtCoordinateEnd()
+            ? childExtent - headerExtent // TODO buggy, should be zero
+            : 0.0;
+
+          // Its layout region (affecting where the next sliver begins layout)
+          // is that given by the child sliver.
+          final layoutExtent = childExtent;
+
+          // The paint origin places this sliver's paint region relative to its
+          // layout region.
+          final paintOrigin = 0.0; // TODO buggy
+
           geometry = SliverGeometry( // TODO review interaction with other slivers
             scrollExtent: geometry.scrollExtent,
-            layoutExtent: childExtent,
-            paintExtent: childExtent,
-            maxPaintExtent: math.max(geometry.maxPaintExtent, headerExtent),
+            layoutExtent: layoutExtent,
+            paintExtent: childExtent, // TODO buggy
+            paintOrigin: paintOrigin,
+            maxPaintExtent: math.max(geometry.maxPaintExtent, paintExtent),
             hasVisualOverflow: geometry.hasVisualOverflow
-              || headerExtent > constraints.remainingPaintExtent,
+              || paintExtent > constraints.remainingPaintExtent,
 
             // The cache extent is an extension of layout, not paint; it controls
             // where the next sliver should start laying out content.  (See
@@ -643,10 +658,6 @@ class _RenderSliverStickyHeaderList extends RenderSliver with RenderSliverHelper
             // affect the cache extent.
             cacheExtent: geometry.cacheExtent,
           );
-
-          headerOffset = _headerAtCoordinateEnd()
-            ? childExtent - headerExtent
-            : 0.0;
         }
       } else {
         // The header's item has [StickyHeaderItem.allowOverflow] false.