feat: variable front page height

lib/scaffold.dart

@@ -146,6 +146,16 @@ class BackdropScaffold extends StatefulWidget {
   /// If null, the color is handled automatically according to the theme.
   final Color frontLayerBackgroundColor;
 
+  /// Fraction of the available height the front layer will occupy,
+  /// when active.  Clamped to (0, 1).
+  ///
+  /// Note the front layer will not fully conceal the back layer when
+  /// this value is less than 1.  A scrim will cover the
+  /// partially concealed back layer if [drawerScrimColor] is provided.
+  ///
+  /// Defaults to 1.0.
+  final double frontLayerActiveFactor;
+
   /// Defines the color for the inactive front layer.
   /// A default opacity of 0.7 is applied to the passed color.
   /// See [inactiveOverlayOpacity].
@@ -271,6 +281,7 @@ class BackdropScaffold extends StatefulWidget {
     this.stickyFrontLayer = false,
     this.animationCurve = Curves.easeInOut,
     this.frontLayerBackgroundColor,
+    double frontLayerActiveFactor = 1.0,
     this.backLayerBackgroundColor,
     this.inactiveOverlayColor = const Color(0xFFEEEEEE),
     this.inactiveOverlayOpacity = 0.7,
@@ -297,6 +308,7 @@ class BackdropScaffold extends StatefulWidget {
     this.drawerEnableOpenDragGesture = true,
     this.endDrawerEnableOpenDragGesture = true,
   })  : assert(inactiveOverlayOpacity >= 0.0 && inactiveOverlayOpacity <= 1.0),
+        frontLayerActiveFactor = frontLayerActiveFactor.clamp(0, 1).toDouble(),
         super(key: key);
 
   @override
@@ -441,21 +453,20 @@ class BackdropScaffoldState extends State<BackdropScaffold>
   Animation<RelativeRect> _getPanelAnimation(
       BuildContext context, BoxConstraints constraints) {
     double backPanelHeight, frontPanelHeight;
-
-    if (widget.stickyFrontLayer &&
-        _backPanelHeight < constraints.biggest.height - _headerHeight) {
+    final availableHeight = constraints.biggest.height - _headerHeight;
+    if (widget.stickyFrontLayer && _backPanelHeight < availableHeight) {
       // height is adapted to the height of the back panel
       backPanelHeight = _backPanelHeight;
       frontPanelHeight = -_backPanelHeight;
     } else {
       // height is set to fixed value defined in widget.headerHeight
-      final height = constraints.biggest.height;
-      backPanelHeight = height - _headerHeight;
+      backPanelHeight = availableHeight;
       frontPanelHeight = -backPanelHeight;
     }
     return RelativeRectTween(
       begin: RelativeRect.fromLTRB(0.0, backPanelHeight, 0.0, frontPanelHeight),
-      end: RelativeRect.fromLTRB(0.0, 0.0, 0.0, 0.0),
+      end: RelativeRect.fromLTRB(
+          0.0, availableHeight * (1 - widget.frontLayerActiveFactor), 0.0, 0.0),
     ).animate(CurvedAnimation(
       parent: controller,
       curve: widget.animationCurve,
@@ -574,6 +585,10 @@ class BackdropScaffoldState extends State<BackdropScaffold>
                 fit: StackFit.expand,
                 children: <Widget>[
                   _buildBackPanel(),
+                  if (isBackLayerConcealed && widget.frontLayerActiveFactor < 1)
+                    Container(
+                        color: widget.drawerScrimColor,
+                        height: _backPanelHeight),
                   PositionedTransition(
                     rect: _getPanelAnimation(context, constraints),
                     child: _buildFrontPanel(context),

pubspec.yaml

@@ -12,7 +12,7 @@ authors:
   - Felix Wortmann <https://github.com/felixwortmann>
 
 environment:
-  sdk: ">=1.19.0 <3.0.0"
+  sdk: ">=2.3.0 <3.0.0"
 
 dependencies:
   flutter: