Common idioms and norms to consider

Scrollbar appearance and behavior
Multi-select
Selectable text
Title bars
Context menus and tooltips
Horizontal button order
Menu bar
Drag and drop


Educate yourself on basic usability principles

Overview

Flutter provides new opportunities to build apps that can
run on mobile, desktop, and the web from a single codebase.
However, with these opportunities, come new challenges.
You want your app to feel familiar to users,
adapting to each platform by maximizing usability and
ensuring a comfortable and seamless experience.
That is, you need to build apps that are not just
multiplatform, but are fully platform adaptive.

There are many considerations for developing platform-adaptive
apps, but they fall into three major categories:

Layout

Input

Idioms and norms