[Linux] Track GTK3 embedder refactors to prepare for GTK4 #194698

Summary

Track incremental refactoring of Flutter's existing GTK3 Linux embedder to improve code structure, maintainability, and compatibility with a future GTK4 implementation.

The goal is to identify changes that can be developed, reviewed, and landed independently while preserving existing GTK3 functionality.

This work supports #94804 (GTK4 support) but does not itself introduce a GTK4 embedder.

Motivation

The Linux embedder currently contains GTK3-specific assumptions throughout its widget, windowing, input, and rendering code.
There is no reason why GTK3 and GTK4 can't co-exist in the tree.

Guidelines

• Preserve existing GTK3 behaviour and supported configurations.
• Avoid public API or ABI changes unless separately justified.
• Prefer existing GTK/GDK terminology and APIs.
• Introduce compatibility helpers only where they provide a clear benefit.
• Avoid unnecessary abstractions or conditional GTK3/GTK4 code throughout the embedder.
• Include appropriate unit tests and regression coverage.
• Keep individual PRs small enough to review independently.

Current PR's

• refactor(linux): introduce GTK3 surface helpers #193943
• fix(linux): handle ATK C linkage with newer GLib headers #193942
• refactor(linux): Extract GTK3 display monitor backend in preparation for GTK4 #195324
• refactor(linux): extract GTK3 pointer, touch and gesture handling #194913
• docs(linux): clarify toolkit-specific windowing contracts #194921

Related work

• Use gtk4 for linux desktop #94804 — GTK4 support for Linux desktop

Completion criteria

This is an ongoing tracking issue. It can be closed when the identified GTK3-side preparatory refactors have been completed or moved into independently tracked work.

GTK4 build support and feature parity are outside the scope of this issue.