test(e2e): deselect by clicking a corner away from the shape

The test clicked the canvas at (10, 10) to deselect. When the click on the
shape before it needed retries, Playwright scrolled the shape into the
container's top-left corner, so the next click landed on the shape and the
selection stayed (seen on the main run of cd5352c and on #973). The new
clickEmptyCanvas helper clicks the corner farthest from the shape.
Reproduced locally by scrolling the shape into the corner: the old click
keeps the selection, the helper clears it.

tests/e2e/workspace.spec.ts

@@ -108,6 +108,25 @@ async function boxOf(locator: Locator) {
     return box
 }
 
+/**
+ * Click the canvas where no shape is, to deselect: the corner of the
+ * container farthest from the shape. A fixed spot near the top-left corner
+ * is not safe, as a click on a shape may scroll it into that corner.
+ */
+async function clickEmptyCanvas(page: Page, shape: Locator) {
+    const container = getIframeContent(page).locator(".geDiagramContainer")
+    const box = await boxOf(container)
+    const shapeBox = await boxOf(shape)
+    const shapeX = shapeBox.x + shapeBox.width / 2
+    const shapeY = shapeBox.y + shapeBox.height / 2
+    await container.click({
+        position: {
+            x: shapeX < box.x + box.width / 2 ? box.width - 20 : 20,
+            y: shapeY < box.y + box.height / 2 ? box.height - 20 : 20,
+        },
+    })
+}
+
 /** draw.io's own Undo button: draw.io sets "disabled" with nothing to undo */
 function drawioUndo(page: Page) {
     return getIframeContent(page).locator('.geSimpleMainMenu a[title="Undo"]')
@@ -4512,9 +4531,7 @@ test.describe("Edge cases", () => {
         })
         await sendMessage(page, "Make this red")
         // Deselected while the diagram exports
-        await canvas
-            .locator(".geDiagramContainer")
-            .click({ position: { x: 10, y: 10 } })
+        await clickEmptyCanvas(page, shape)
         await expect(chip).toHaveCount(0)
         await page.evaluate(() => (window as any).__releaseExports())
         await waitForCompleteCount(page, 2)