UsdLayerManager/openspec/changes/multi-viewport-support/proposal.md

Why

Multi-viewport support is a standard feature in DCC tools (Maya, Houdini, Blender) that lets users view the same scene from different camera angles simultaneously. Currently the app has a single ViewportPanel — users cannot compare views (e.g., free camera + a USD camera prim, or top/orthographic + perspective) without switching back and forth. This limits scene inspection, layout work, and side-by-side comparison during shading and lighting iteration.

What Changes

• Introduce a viewport layout system that manages 1–4 viewport tiles within a single ImGui window area
• Each tile has its own ViewportCamera, UsdSceneRenderer, selection highlight set, and per-viewport settings (grid, AA, background color, bounding box mode)
• Selection is shared across all viewports (pick in any viewport, all viewports highlight)
• The main ViewportPanel becomes a container that holds N child viewports (N=1 default, up to 4)
• Layout presets: single, 2-split horizontal/vertical, 4-quadrant
• Per-viewport camera toolbar remains (camera selector, render delegate, view options) but is compact (icon-only) to conserve space
• The manipulator gizmo is only active in the focused viewport (the one that last received a click)
• Space-key maximize: pressing Space while hovering over a tile temporarily maximizes that tile to fill the entire viewport area, hiding dividers and other tiles. Pressing Space again restores the previous multi-tile layout (identical to Maya's viewport maximize behavior)
• BREAKING: Application's m_viewportPanel member type changes (it remains ViewportPanel but ViewportPanel switches from a single renderer to a container)

Capabilities

New Capabilities

• viewport-layout: Layout management — splitting, merging, preset switching, drag-resize dividers, Space-key maximize/restore, persistence of layout choice
• per-viewport-camera: Each tile has an independent ViewportCamera, camera selector dropdown, and free/USD camera switching
• per-viewport-settings: Per-tile grid toggle, AA, background color, bounding box mode, render delegate selection (independent per viewport)
• shared-selection: Single selection model across all viewports — picking in any tile updates all others and propagates to SceneHierarchyPanel / PropertyPanel
• focused-viewport-manipulator: Transform gizmo only renders and accepts input in the most recently clicked viewport tile

Modified Capabilities

• (None — imgui-docking is purely an ImGui infrastructure change; the new viewport system builds on top of it and does not alter its spec)

Impact

• src/ui/ViewportPanel.h/.cpp: Rewritten from a single-viewport class to a container that owns N ViewportTile instances. Public API surface changes — SetSelectedPrimPath, SetStage, GetCamera, GetRenderer remain but are forwarded to the active/focused tile or broadcast to all tiles.
• src/core/UsdSceneRenderer.h/.cpp: No changes needed (each tile owns its own UsdSceneRenderer already).
• src/core/ViewportCamera.h/.cpp: No changes needed (each tile owns its own ViewportCamera already).
• src/ui/Application.h/.cpp: Wire up OnPrimPicked / OnPrimsPickedRect as before on the container; the container relays selection to the SceneHierarchyPanel / PropertyPanel.
• New files: src/ui/ViewportTile.h/.cpp — the per-tile rendering logic extracted from ViewportPanel::Render().
• New files: src/ui/ViewportLayout.h/.cpp — layout splitting/merge logic, divider drag, layout persistence.
• Memory: N UsdSceneRenderer instances means N FBO draw targets. For N≤4 and typical 1080p viewports this is ~200 MB worst-case — acceptable for a DCC tool.