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openspec/changes/multi-viewport-support/specs/viewport-layout/spec.md

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+## ADDED Requirements
+
+### Requirement: Layout presets
+The viewport container SHALL support switching between four layout presets at any time: Single (1 tile), HSplit (2 tiles side-by-side), VSplit (2 tiles top-bottom), and Quad (4 tiles in a 2×2 grid).
+
+#### Scenario: Switch layout from Single to HSplit
+- **WHEN** the user selects "Horizontal Split" from the layout menu
+- **THEN** the viewport splits into two equal-sized tiles side-by-side
+
+#### Scenario: Switch layout from HSplit to Single
+- **WHEN** the user selects "Single" from the layout menu
+- **THEN** the two tiles merge back into a single tile, and the focused tile's camera/settings are preserved
+
+#### Scenario: Switch from HSplit to Quad
+- **WHEN** the user switches from HSplit to Quad
+- **THEN** four tiles are created in a 2×2 grid, filling the viewport area
+
+### Requirement: Draggable dividers
+The divider line between tiles SHALL be draggable to resize adjacent tiles. Dividers SHALL be 6px wide with visual hover feedback.
+
+#### Scenario: Drag horizontal divider
+- **WHEN** the user clicks and drags the divider between two tiles
+- **THEN** the divider follows the mouse, and tiles resize proportionally
+
+#### Scenario: Drag divider to edge
+- **WHEN** the user drags a divider to within 10% of the viewport edge
+- **THEN** the divider snaps back to the 10% boundary to prevent tiles from becoming too small (minimum 100px per dimension)
+
+### Requirement: Layout applies to the entire viewport area
+When the layout changes, all tiles SHALL be redistributed to fill the entire available viewport panel content region. No gaps between tiles.
+
+#### Scenario: Resize main window with Quad layout
+- **WHEN** the user resizes the main application window
+- **THEN** all four tiles resize proportionally to maintain their relative sizes and fill the viewport content area
+
+### Requirement: Space-key maximize restores previous layout
+When maximized, pressing Space again SHALL restore the exact layout and divider positions from before maximization.
+
+#### Scenario: Maximize and restore preserves layout
+- **WHEN** the user has an HSplit layout with divider at 0.3, then hovers tile B and presses Space, then presses Space again
+- **THEN** the viewport returns to HSplit layout with both tiles visible and the divider at 0.3
+
+### Requirement: Escape restores maximized viewport
+When maximized, pressing Escape SHALL restore the previous layout (same behavior as Space toggle).
+
+#### Scenario: Escape during maximize
+- **WHEN** the viewport is maximized and the user presses Escape
+- **THEN** the previous multi-tile layout is restored
+
+### Requirement: Space requires tile hover in multi-tile layouts
+In Single layout (only 1 tile), Space SHALL be a no-op — there is nothing to maximize.
+
+#### Scenario: Space ignored in Single layout
+- **WHEN** the viewport is already in Single layout and the user presses Space
+- **THEN** the viewport remains unchanged in Single layout