Prefer span notation when resizing flow grid cells (#6687)

**Problem:**

When resizing a grid cell for an in-flow element, `span` notation should
be prefered over explicit pins.

**Fix:**

If the resize happens on a right/bottom edge and the element is in flow,
apply the `span` notation.

**Manual Tests:**
I hereby swear that:

- [x] I opened a hydrogen project and it loaded
- [x] I could navigate to various routes in Play mode

Fixes #6686

editor/src/components/canvas/canvas-strategies/strategies/grid-resize-element-strategy.spec.browser2.tsx

@@ -849,6 +849,107 @@ export var storyboard = (
         })
       }
     })
+    it('uses spans for flow elements', async () => {
+      const editor = await renderTestEditorWithCode(
+        makeProjectCodeWithCustomPlacement({ gridColumn: 'auto', gridRow: 'auto' }),
+        'await-first-dom-report',
+      )
+
+      // enlarge to the right, spans in flow
+      {
+        await runCellResizeTest(
+          editor,
+          'column-end',
+          gridCellTargetId(EP.fromString('sb/grid'), 1, 3),
+          EP.fromString('sb/grid/cell'),
+        )
+
+        const { gridRowStart, gridRowEnd, gridColumnStart, gridColumnEnd } =
+          editor.renderedDOM.getByTestId('cell').style
+        expect({ gridRowStart, gridRowEnd, gridColumnStart, gridColumnEnd }).toEqual({
+          gridColumnEnd: 'auto',
+          gridColumnStart: 'span 3',
+          gridRowEnd: 'auto',
+          gridRowStart: 'auto',
+        })
+      }
+
+      // enlarge to the bottom, still spans in flow
+      {
+        await runCellResizeTest(
+          editor,
+          'row-end',
+          gridCellTargetId(EP.fromString('sb/grid'), 4, 3),
+          EP.fromString('sb/grid/cell'),
+        )
+
+        const { gridRowStart, gridRowEnd, gridColumnStart, gridColumnEnd } =
+          editor.renderedDOM.getByTestId('cell').style
+        expect({ gridRowStart, gridRowEnd, gridColumnStart, gridColumnEnd }).toEqual({
+          gridColumnEnd: 'auto',
+          gridColumnStart: 'span 3',
+          gridRowEnd: 'auto',
+          gridRowStart: 'span 4',
+        })
+      }
+
+      // shrink from the right, still spans in flow
+      {
+        await runCellResizeTest(
+          editor,
+          'column-end',
+          gridCellTargetId(EP.fromString('sb/grid'), 4, 2),
+          EP.fromString('sb/grid/cell'),
+        )
+
+        const { gridRowStart, gridRowEnd, gridColumnStart, gridColumnEnd } =
+          editor.renderedDOM.getByTestId('cell').style
+        expect({ gridRowStart, gridRowEnd, gridColumnStart, gridColumnEnd }).toEqual({
+          gridColumnEnd: 'auto',
+          gridColumnStart: 'span 2',
+          gridRowEnd: 'auto',
+          gridRowStart: 'span 4',
+        })
+      }
+
+      // shrink from the bottom, still spans in flow
+      {
+        await runCellResizeTest(
+          editor,
+          'row-end',
+          gridCellTargetId(EP.fromString('sb/grid'), 3, 2),
+          EP.fromString('sb/grid/cell'),
+        )
+
+        const { gridRowStart, gridRowEnd, gridColumnStart, gridColumnEnd } =
+          editor.renderedDOM.getByTestId('cell').style
+        expect({ gridRowStart, gridRowEnd, gridColumnStart, gridColumnEnd }).toEqual({
+          gridColumnEnd: 'auto',
+          gridColumnStart: 'span 2',
+          gridRowEnd: 'auto',
+          gridRowStart: 'span 3',
+        })
+      }
+
+      // shrink from the top, it spans but now is pinned
+      {
+        await runCellResizeTest(
+          editor,
+          'row-start',
+          gridCellTargetId(EP.fromString('sb/grid'), 2, 2),
+          EP.fromString('sb/grid/cell'),
+        )
+
+        const { gridRowStart, gridRowEnd, gridColumnStart, gridColumnEnd } =
+          editor.renderedDOM.getByTestId('cell').style
+        expect({ gridRowStart, gridRowEnd, gridColumnStart, gridColumnEnd }).toEqual({
+          gridColumnEnd: 'auto',
+          gridColumnStart: 'span 2',
+          gridRowEnd: '4',
+          gridRowStart: 'span 2',
+        })
+      }
+    })
   })
 })
 

editor/src/components/canvas/canvas-strategies/strategies/grid-resize-element-strategy.ts

@@ -27,8 +27,8 @@ import {
   emptyStrategyApplicationResult,
   strategyApplicationResult,
 } from '../canvas-strategy-types'
-import type { InteractionSession } from '../interaction-state'
-import { getCommandsForGridItemPlacement } from './grid-helpers'
+import type { GridResizeEdge, InteractionSession } from '../interaction-state'
+import { getCommandsForGridItemPlacement, isAutoGridPin } from './grid-helpers'
 import { resizeBoundingBoxFromSide } from './resize-helpers'
 
 export const gridResizeElementStrategy: CanvasStrategyFactory = (
@@ -138,6 +138,7 @@ export const gridResizeElementStrategy: CanvasStrategyFactory = (
           'start',
           elementGridPropertiesFromProps.gridColumnEnd,
           gridPropsNumeric.gridColumnEnd,
+          interactionSession.activeControl.edge,
         ),
         gridColumnEnd: normalizePositionAfterResize(
           elementGridPropertiesFromProps.gridColumnEnd,
@@ -146,6 +147,7 @@ export const gridResizeElementStrategy: CanvasStrategyFactory = (
           'end',
           elementGridPropertiesFromProps.gridColumnStart,
           gridPropsNumeric.gridColumnStart,
+          interactionSession.activeControl.edge,
         ),
         gridRowStart: normalizePositionAfterResize(
           elementGridPropertiesFromProps.gridRowStart,
@@ -154,6 +156,7 @@ export const gridResizeElementStrategy: CanvasStrategyFactory = (
           'start',
           elementGridPropertiesFromProps.gridRowEnd,
           gridPropsNumeric.gridRowEnd,
+          interactionSession.activeControl.edge,
         ),
         gridRowEnd: normalizePositionAfterResize(
           elementGridPropertiesFromProps.gridRowEnd,
@@ -162,6 +165,7 @@ export const gridResizeElementStrategy: CanvasStrategyFactory = (
           'end',
           elementGridPropertiesFromProps.gridRowStart,
           gridPropsNumeric.gridRowStart,
+          interactionSession.activeControl.edge,
         ),
       }
 
@@ -211,23 +215,47 @@ function getNewGridPropsFromResizeBox(
   }
 }
 
-// After a resize happens and we know the numerical grid positioning of the new bounds,
-// return a normalized version of the new position so that it respects any spans that
-// may have been there before the resize, and/or default it to 'auto' when it would become redundant.
+/*
+    After a resize happens and we know the numerical grid positioning of the new bounds,
+    return a normalized version of the new position so that it respects any spans that
+    may have been there before the resize, and/or default it to 'auto' when it would become redundant.
+    If the positions match a flow configuration, give priority to span notation.
+*/
 function normalizePositionAfterResize(
   position: GridPositionOrSpan | null,
   resizedPosition: GridPositionValue,
   size: number, // the number of cols/rows the cell occupies
   bound: 'start' | 'end',
   counterpart: GridPositionOrSpan | null,
   counterpartResizedPosition: GridPositionValue,
+  edge: GridResizeEdge,
 ): GridPositionOrSpan | null {
-  if (isGridSpan(position)) {
+  function isFlowResizeOnBound(
+    wantedBound: 'start' | 'end',
+    flowStart: GridPositionOrSpan | null,
+    flowEnd: GridPositionOrSpan | null,
+  ): boolean {
+    return (
+      (edge === 'column-end' || edge === 'row-end') &&
+      bound === wantedBound &&
+      (isGridSpan(flowStart) || isAutoGridPin(flowStart) || flowStart == null) &&
+      (isAutoGridPin(flowEnd) || flowEnd == null)
+    )
+  }
+
+  const isFlowStart = isFlowResizeOnBound('start', position, counterpart)
+  if (isFlowStart || isGridSpan(position)) {
     if (size === 1) {
       return cssKeyword('auto')
     }
     return gridSpanNumeric(size)
   }
+
+  const isFlowEnd = isFlowResizeOnBound('end', counterpart, position)
+  if (isFlowEnd) {
+    return cssKeyword('auto')
+  }
+
   if (
     isGridSpan(counterpart) &&
     counterpartResizedPosition.numericalPosition === 1 &&