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+# Phase 4: Heatmap Interaction - Context
+
+**Gathered:** 2026-04-08
+**Status:** Ready for planning
+
+<domain>
+## Phase Boundary
+
+Add click-through navigation from heatmap day cells to Kimai's timesheet view, and a project filter dropdown that re-fetches and re-renders the heatmap without page reload.
+
+</domain>
+
+<decisions>
+## Implementation Decisions
+
+### Click Navigation
+- Clicking any day cell navigates to Kimai's timesheet list filtered by that date (`/en/timesheet/?daterange=YYYY-MM-DD`)
+- Empty cells (no data) are clickable too — user may want to add time for that day
+- Pointer cursor + subtle hover highlight (opacity change) on all cells as click affordance
+- If a project filter is active, preserve it in the timesheet URL (`&projects[]=N`)
+
+### Project Filter
+- Filter dropdown positioned next to the heatmap SVG (right side), using the spare horizontal space since the heatmap isn't full card width
+- Projects only — no activity filter (API already supports `?project=N`)
+- Default state: "All Projects" selected, no filter applied
+- Selecting a project fetches from API with `?project=N`, re-renders heatmap in place; color scale recalculates for filtered data
+
+### Testing Strategy
+- Vitest tests for click navigation: verify click handler sets `window.location.href` to correct Kimai URL pattern
+- Vitest tests for filter dropdown: verify dropdown renders, selecting fires fetch with correct query param, heatmap re-renders with new data
+- Fixture data matching existing HeatmapData shape from prior tests
+
+### Claude's Discretion
+- Exact Kimai timesheet URL format (verify against Kimai source)
+- Filter dropdown styling details (should use Tabler/Kimai form classes)
+- How to populate the project list (new API endpoint or extend existing one)
+
+</decisions>
+
+<code_context>
+## Existing Code Insights
+
+### Reusable Assets
+- `renderHeatmap(container, data, config)` in `assets/src/heatmap.ts` — main rendering function, can be called again with new data for filter updates
+- `HeatmapData` / `DayEntry` types in `assets/src/types.ts` — shared data shape
+- `HeatmapController::data()` already accepts `?project` query parameter
+- `HeatmapService::getDailyAggregation()` already supports `?int $projectId` parameter
+
+### Established Patterns
+- d3 selections for SVG rendering with data binding
+- Tooltip positioning via fixed-position div (mouseenter/mouseleave)
+- IIFE format with `KimaiHeatmap` global, `kimai.initialized` event for deferred init
+- Tabler CSS variables for theming (`--tblr-bg-surface`, `--tblr-body-color`, etc.)
+
+### Integration Points
+- `data-url` attribute on container div passes API endpoint to JS
+- Widget template at `Resources/views/widget/heatmap.html.twig` with Tabler card embed
+- CSS at `Resources/public/heatmap.css`
+- Route `heatmap_data` resolves to `/heatmap/data`
+
+</code_context>
+
+<specifics>
+## Specific Ideas
+
+- User wants filter next to the heatmap (not in header or below) since the SVG doesn't fill the full card width — use the horizontal space to the right
+
+</specifics>
+
+<deferred>
+## Deferred Ideas
+
+None — discussion stayed within phase scope.
+
+</deferred>