docs: note scrollbar differences between Material and Cupertino (#12899)

Description of what this PR is changing or adding, and why:

Adds a short note in the Scrolling section describing visual scrollbar
differences
between Material and Cupertino platforms. This clarifies platform
adaptation
behavior and addresses the request in #4306.

Issues fixed by this PR (if any):

Fixes #4306

PRs or commits this PR depends on (if any):

None.

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---------

Co-authored-by: Shams Zakhour <[email protected]>
Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: 3 people

src/content/ui/adaptive-responsive/platform-adaptations.md

@@ -149,6 +149,19 @@ On **iOS**, scrolling past the edge of a scrollable
 [overscroll glow indicator]: {{site.api}}/flutter/widgets/GlowingOverscrollIndicator-class.html
 [overscrolls]: {{site.api}}/flutter/widgets/BouncingScrollPhysics-class.html
 
+### Scrollbars
+
+On **Material-based platforms** (such as Android and web),
+scrollbars are typically visible during scrolling
+and may remain visible depending on the platform and theme.
+
+On **Cupertino-based platforms** (such as iOS),
+scrollbars are more minimal and generally only appear briefly
+while the user is actively scrolling, fading out when interaction stops.
+
+This difference reflects each platform’s visual conventions
+and helps maintain a native look and feel across devices.
+
 ### Momentum
 
 On **iOS**,