Graphics: acquire D3D12 back buffer per-present (resize/F-11 fix)

[Kheartz]

This issue predated my work on the d3d12 view. When Init is called, the back buffers were cached, but these are usually recreated whenever the window is resized--making references to them invalid.

The backend cached the swapchain back buffers for the process lifetime, so the game's ResizeBuffers failed (DXGI_ERROR_INVALID_CALL) and UE4 crashed on any fullscreen/resolution change. Acquire the back buffer fresh in Begin() and release it in End() so no reference is held across frames.

Summary by CodeRabbit

• Bug Fixes

  • Improved frame rendering reliability by handling render targets per frame instead of keeping them across frames.
  • Added safer cleanup so back-buffer resources are released correctly, helping avoid issues during resize or swapchain recreation.
  • Updated frame presentation flow to better guard against initialization failures and invalid back-buffer usage.

• New Features

  • Added swapchain access controls so the graphics backend can be pointed to a recreated swapchain.

[coderabbitai]

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: e70624bd-4809-4dde-9eee-d55f6b732d97

📥 Commits

Reviewing files that changed from the base of the PR and between 3f973eb and cbe53e7.

📒 Files selected for processing (2)

• code/framework/src/graphics/backend/d3d12.cpp
• code/framework/src/graphics/backend/d3d12.h

Walkthrough

The D3D12 backend now acquires the swapchain back buffer each frame, creates the RTV during Begin, and releases that buffer in End and Shutdown. The header removes stored render-target ownership and adds swapchain accessors.

Changes

D3D12 back-buffer lifecycle

Layer / File(s)	Summary
Frame state and swapchain accessors code/framework/src/graphics/backend/d3d12.h	FrameContext drops _mainRenderTargetResource, _barrier is initialized at backend scope, _currentBackBuffer is added, and GetSwapChain() / SetSwapChain() are declared.
Init-time RTV setup and back-buffer acquisition code/framework/src/graphics/backend/d3d12.cpp	Init only stores reserved RTV descriptor handles, and Begin acquires the current swapchain buffer, creates its RTV, and transitions that buffer for rendering.
End and shutdown release code/framework/src/graphics/backend/d3d12.cpp	End returns early when _currentBackBuffer is absent, performs the present transition and submission, and releases _currentBackBuffer; Shutdown clears any remaining _currentBackBuffer and stops releasing frame-context render-target resources.

Sequence Diagram(s)

sequenceDiagram
  participant D3D12Backend
  participant IDXGISwapChain3
  participant FrameContext
  D3D12Backend->>FrameContext: assign reserved main render target descriptor in Init()
  D3D12Backend->>IDXGISwapChain3: GetCurrentBackBufferIndex()
  D3D12Backend->>IDXGISwapChain3: GetBuffer(index, _currentBackBuffer)
  D3D12Backend->>D3D12Backend: CreateRenderTargetView(_currentBackBuffer, frameContext._mainRenderTargetDescriptor)
  D3D12Backend->>D3D12Backend: transition _currentBackBuffer for rendering
  D3D12Backend->>D3D12Backend: transition _currentBackBuffer for present
  D3D12Backend->>D3D12Backend: release _currentBackBuffer in End()

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Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Suggested reviewers

• zpl-zak

Poem

Hop hop, I fetched the buffer bright,
Reserved the RTV, set it right.
I render, present, then let it go,
So every frame can start the show. 🐰

🚥 Pre-merge checks | ✅ 4 | ❌ 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 (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly describes the main D3D12 back-buffer acquisition change and the resize/fullscreen fix.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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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 current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In `@code/framework/src/graphics/backend/d3d12.h`:
- Around line 62-64: `SetSwapChain` currently only replaces `_swapChain` and
leaves the frame-state derived from `Init()` stale. Update the
`D3D12Backend::SetSwapChain` flow to rebuild swapchain-dependent state when a
new `IDXGISwapChain3` is assigned: refresh `_frameBufferCount`,
resize/reinitialize `_frameContext`, and ensure any RTV/descriptor resources
tied to the old buffer count are recreated or validated before `Begin()` uses
them.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

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

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 23cdfd0f-e3a7-4d96-9bc6-0b6e0500af19

📥 Commits

Reviewing files that changed from the base of the PR and between 6a7378e and 3f973eb.

📒 Files selected for processing (2)

• code/framework/src/graphics/backend/d3d12.cpp
• code/framework/src/graphics/backend/d3d12.h