tweaked slider transparency for flat design mode

HISTORY.md

@@ -12,7 +12,7 @@
     * libraries no longer rely on the JSF primitive, project may need to re-import their libraries to run without having to enable JS extensions
     * retired Leap Motion library, took out Hummingbird library (get the current one from Birdbrain)
     * display blocks with their error messages for custom blocks, thanks, Michael!
-    * made scrollbars thinner by default
+    * made scrollbars thinner by default and slightly transparent in flat design mode
 * **Notable Fixes:**
     * made scrollbars in the wardrobe and jukebox more responsive
     * fixed centering of menus, thanks, Brian Broll!
@@ -24,6 +24,7 @@
 ### 2021-07-09
 * extensions: whitelisted 'https://snap.berkeley.edu/'
 * morphic: made scrollbars thinner by default
+* morphic: tweaked slider transparency for flat design mode
 
 ### 2021-07-08
 * objects: added category labels to unified palette

src/morphic.js

@@ -7040,12 +7040,13 @@ SliderMorph.prototype.init = function (
     this.offset = null;
     this.button = new SliderButtonMorph();
     this.button.isDraggable = false;
+    this.button.alpha = MorphicPreferences.isFlat ? 0.7 : 1;
     this.button.color = new Color(200, 200, 200);
     this.button.highlightColor = new Color(210, 210, 255);
     this.button.pressColor = new Color(180, 180, 255);
     SliderMorph.uber.init.call(this, orientation);
     this.add(this.button);
-    this.alpha = 0.3;
+    this.alpha = MorphicPreferences.isFlat ? 0 : 0.3;
     this.color = color || new Color(0, 0, 0);
     this.setExtent(new Point(20, 100));
     this.fixLayout();