cmd/worklog: change amendment semantics to allow partial changes

Previously, amendments were a complete change of the data field. The new
behaviour is to merge the amendment into the existing data, with the option to
remove existing fields in data by assigning null to the key.

cmd/worklog/dashboard.go

@@ -12,6 +12,7 @@ import (
 	"fmt"
 	"io"
 	"log/slog"
+	"maps"
 	"net/http"
 	"net/url"
 	"reflect"
@@ -663,7 +664,7 @@ func (d *daemon) applyAmendments(ctx context.Context, e worklog.Event) []worklog
 			//
 			// a:     ~~~~~~~~~~~
 
-			e.Data = r.Data
+			e.Data = mergeData(e.Data, r.Data)
 			return append(events, e)
 		case r.overlapLeftOf(w):
 			// Replace left data and truncate rightward.
@@ -678,7 +679,7 @@ func (d *daemon) applyAmendments(ctx context.Context, e worklog.Event) []worklog
 
 			left := e
 			left.End = r.Start
-			left.Data = r.Data
+			left.Data = mergeData(e.Data, r.Data)
 			events = append(events, left)
 			e.Start = r.End
 		case r.overlapRightOf(w):
@@ -694,7 +695,7 @@ func (d *daemon) applyAmendments(ctx context.Context, e worklog.Event) []worklog
 
 			right := e
 			right.Start = r.End
-			right.Data = r.Data
+			right.Data = mergeData(e.Data, r.Data)
 			e.End = r.Start
 			if e.End.Before(e.Start) {
 				events = append(events, e)
@@ -720,7 +721,7 @@ func (d *daemon) applyAmendments(ctx context.Context, e worklog.Event) []worklog
 			middle := e
 			middle.Start = r.Start
 			middle.End = r.End
-			middle.Data = r.Data
+			middle.Data = mergeData(e.Data, r.Data)
 			events = append(events, middle)
 
 			e.Start = r.End
@@ -742,6 +743,33 @@ func (d *daemon) applyAmendments(ctx context.Context, e worklog.Event) []worklog
 	return events
 }
 
+// mergeData adds elements of src into a clone of dst recursively, removing any
+// elements that are nil in src and replacing elements in dst with elements in
+// src if they have the same key.
+func mergeData(dst, src map[string]any) map[string]any {
+	if dst == nil {
+		dst = make(map[string]any)
+	} else {
+		dst = maps.Clone(dst)
+	}
+	for k, v := range src {
+		switch {
+		case v == nil:
+			delete(dst, k)
+		case is[map[string]any](dst[k]) && is[map[string]any](v):
+			dst[k] = mergeData(dst[k].(map[string]any), v.(map[string]any))
+		default:
+			dst[k] = v
+		}
+	}
+	return dst
+}
+
+func is[T any](v any) bool {
+	_, ok := v.(T)
+	return ok
+}
+
 type worklogEvent worklog.Event
 
 func (e worklogEvent) start() time.Time { return e.Start }