fix(v3,linux): implement native frameless window resize

[BenAicholtz]

Summary

linuxWebviewWindow.startResize was a no-op stub, so wails:resize:<edge> messages sent by the runtime's drag.ts were dispatched but never acted upon. Frameless windows on Linux could only be dragged, not resized from edges/corners.

This PR wires both CGo backends up to the underlying GTK resize APIs.

Changes

• GTK3 (linux_cgo.go): map the eight edge strings to GdkWindowEdge and call gtk_window_begin_resize_drag, using the mouse state captured by the existing button-press-event handler.
• GTK4 (linux_cgo_gtk4.go): same mapping to GdkSurfaceEdge, dispatched through the beginWindowResize C helper that was already defined in linux_cgo_gtk4.c / .h but never called.
• webview_window_linux.go: remove the // FIXME: what do we need to do here? stub so the per-backend implementations are the only definitions.

The pattern mirrors the existing startDrag implementations on both backends — the w.drag state populated by mousedown is reused with no new plumbing.

Why this matters

• Frameless windows on Linux are common for PiP / floating video overlays / custom chromeless apps. Without resize support the only workarounds are (a) a non-frameless window with system decorations, or (b) a manual JS implementation that round-trips SetSize/SetPosition over the IPC channel per mousemove — visibly janky compared to native resize.
• The C helper and the --wails-resize / wails:resize: runtime plumbing already exist; this PR only wires them together.

Fixes

• [V3]Linux Mint Frameless not work #4976

Test plan

• Build a frameless window app on GTK3 (default): drag from each of 8 edges/corners → window resizes smoothly and stops on mouseup.
• Build the same app with -tags gtk4: drag from each of 8 edges/corners → same behavior.
• Confirm that passing an unknown border string returns a clear error (unknown resize border: %q) rather than silently doing nothing.
• Verify the existing startDrag behavior is unchanged.

🤖 Generated with Claude Code

Summary by CodeRabbit

• New Features
  • Added window resize-drag functionality for Linux applications, allowing users to resize windows by dragging from any border edge (north, south, east, west, and corner edges).

[coderabbitai]

Walkthrough

Added window resize functionality for Linux by implementing the startResize method in both GTK3 and GTK4 platform-specific implementations, with border string validation and drag state integration. Removed the previous stub method.

Changes

Cohort / File(s)	Summary
GTK3 Implementation v3/pkg/application/linux_cgo.go	Added gdkEdgeForBorder lookup table mapping Wails border strings to GTK edges and implemented startResize method that validates border input and initiates window resize via C.gtk_window_begin_resize_drag using stored drag state.
GTK4 Implementation v3/pkg/application/linux_cgo_gtk4.go	Replaced placeholder comment with functional implementation: added gdkSurfaceEdgeForBorder map for border-to-edge translation and implemented startResize method using C.beginWindowResize with validated border mapping and drag state.
Stub Removal v3/pkg/application/webview_window_linux.go	Removed the empty startResize stub method that previously had no implementation.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Suggested labels

Bug, Linux, v3, v3-alpha, size:M, lgtm

Suggested reviewers

• leaanthony

Poem

🐰 A window dancing to resize,
With borders mapped and drag-state wise,
GTK3 and four now play along,
The resize logic strong and long!
✨ hops with joy

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	Title clearly summarizes the main change: implementing native frameless window resize on Linux, which is the core objective of the PR.
Description check	✅ Passed	Description is comprehensive and well-structured, covering the problem, solution details for both GTK backends, motivation, and test plan, though required template sections are not fully checked.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

Verify each finding against the current code and only fix it if needed.

Inline comments:
In `@v3/pkg/application/linux_cgo.go`:
- Around line 1770-1801: The build breaks because linux_purego.go lacks the
startResize(border string) error method required by the webviewWindowImpl
interface; add a method named startResize on the linuxWebviewWindow type in
linux_purego.go that matches the signature (func (w *linuxWebviewWindow)
startResize(border string) error) and either implement equivalent purego
behavior or return a no-op nil/error indicating unsupported resize-drag;
alternatively, add a no-op stub with that same signature to
webview_window_linux.go so purego builds satisfy the interface (ensure the
symbol name startResize and receiver type linuxWebviewWindow match existing
code).

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

Run ID: d6a7590e-6275-4c49-88d1-80335e90794a

📥 Commits

Reviewing files that changed from the base of the PR and between 25403ff and b4e8d98.

📒 Files selected for processing (3)

• v3/pkg/application/linux_cgo.go
• v3/pkg/application/linux_cgo_gtk4.go
• v3/pkg/application/webview_window_linux.go

💤 Files with no reviewable changes (1)

• v3/pkg/application/webview_window_linux.go

[leaanthony]

Thanks for taking this on!