# display-scaling Specification

## Purpose
Keeps the application rendering crisp on displays with integer or fractional scaling by converting between logical and physical coordinates.

## Requirements

### Requirement: Fractional scale is applied when available
The system SHALL use the compositor-provided fractional preferred scale for rendering when the fractional scale protocol is available.

#### Scenario: Fractional display
- **WHEN** the compositor sends a preferred scale of 1.5
- **THEN** the render buffer is created at 1.5 times the logical size
- **AND** the buffer is displayed at the logical size

### Requirement: Integer scale is used as fallback
The system SHALL use the output's integer scale when fractional scale is unavailable.

#### Scenario: No fractional protocol
- **WHEN** the compositor does not support fractional scaling
- **THEN** the render buffer uses the output integer scale
- **AND** the buffer is displayed at the logical size

### Requirement: Input coordinates remain logical
The system SHALL report mouse positions and grid cells in logical coordinates regardless of the active scale.

#### Scenario: Scaled click
- **WHEN** the user clicks on a scaled display
- **THEN** the selected grid cell matches the visual location of the cursor

### Requirement: Rendering uses physical resolution
The system SHALL render all content at physical resolution so that lines and cells stay sharp at the active scale.

#### Scenario: Sharp cells
- **WHEN** the application runs on a 2x scaled display
- **THEN** cells are drawn with 2 physical pixels per logical pixel

### Requirement: Scale changes trigger re-render
The system SHALL recreate buffers and re-render when the active scale changes.

#### Scenario: Scale change
- **WHEN** the compositor changes the preferred scale
- **THEN** the application updates its buffers
- **AND** subsequent frames use the new scale