msglist: Fix double-padding of bottom inset

The user-visible effect was pretty small, so I didn't debug it fully
in PR #3: on my iPhone, before this commit, scrolling all the way
down in the message list left about a centimeter, maybe less,
between the bottom of the latest message and the top of the
compose-box placeholder.

Now, on my iPhone, scrolling down puts the bottom edge of the latest
message flush with the top of the compose-box placeholder, as has
been the case on devices without insets. I'm not sure if that's
quite the effect we want, but if we want to add padding, we should
do it on purpose.

Addressing Greg's TODO in 88f3b73 would have prevented this bug
from sneaking in; I'll take care of that next:

  // TODO have MessageListPage adjust the MediaQuery so we don't
  //   have to worry about this here

Author: chrisbobbe

lib/widgets/message_list.dart

@@ -93,6 +93,24 @@ class _MessageListState extends State<MessageList> {
     final length = model!.messages.length;
     assert(model!.contents.length == length);
     return StickyHeaderListView.builder(
+        // If the `padding` param is omitted or null, StickyHeaderListView pads
+        // the content area (the part that moves when you scroll) with
+        // `MediaQueryData.padding`. It inherits this behavior from
+        // BoxScrollView:
+        //   https://github.com/flutter/flutter/blob/3.7.0-1.2.pre/packages/flutter/lib/src/widgets/scroll_view.dart#L665-L687
+        //
+        // A non-ancestor (the compose box) pads the bottom inset, so the
+        // ambient `MediaQueryData.padding` might be nonzero. So, prevent double
+        // padding at the bottom by supplying a `padding` param.
+        //
+        // (A non-ancestor, the app bar, pads the top inset, but we don't
+        // need the remedy of a non-null `padding` for the top because the
+        // ambient `MediaQueryData.padding` has `top: 0` thanks to Scaffold
+        // setting that on its `body`, since it knows about the app bar:
+        //   https://github.com/flutter/flutter/blob/3.7.0-1.2.pre/packages/flutter/lib/src/material/scaffold.dart#L2778
+        // )
+        padding: const EdgeInsets.all(0),
+
         itemCount: length,
         // Setting reverse: true means the scroll starts at the bottom.
         // Flipping the indexes (in itemBuilder) means the start/bottom