Let the slider keep the drag the browser wanted to take

Dragging the brightness slider on the wall panel set the handle and
published nothing; tapping a point on the track worked. The control is a
native range input laid transparent over the drawn track, and only the
release publishes — but the input never said the drag was its own. On a
touch panel a sideways swipe is a pan, or a back-navigation, so the
browser took the pointer over mid-drag and ended it in `pointercancel`.
The value had followed the finger and was never sent.

`touch-action: none`, as the colour disk beside it has always had. The
release also answers `pointercancel` and `lostpointercapture` now, which
covers a mouse let go outside the input and leaves no way for a draft to
sit there unpublished.

Two checks: a drag across the track reaches the engine, and the input
still owns its gesture. The second fails against a build without the
CSS, which is what makes it worth having.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012n6CehUsHYYUXJ48rxaD18

frontend/tests/slider-drag.spec.ts

@@ -0,0 +1,144 @@
+import { expect, test } from "@playwright/test"
+
+import { api, apiPage, deleteAll } from "./utils/api"
+
+/**
+ * A slider publishes what a *drag* leaves it on, not only what a tap picks.
+ *
+ * The control is a native range input laid transparent over the drawn track,
+ * and only the release publishes — so every way a drag can end has to reach
+ * the release. On a touch panel the browser will happily read a sideways swipe
+ * as a pan and take the pointer over mid-drag, which ends the drag in
+ * `pointercancel`; the value then followed the finger and was never sent.
+ * `touch-action: none` is what stops that, and this is what notices if it goes.
+ */
+const flowName = `test_slider_${Date.now().toString(36)}`
+const dashboard = `${flowName}_d`
+
+test.use({ hasTouch: true, storageState: "playwright/.auth/user.json" })
+test.describe.configure({ mode: "serial" })
+
+test.beforeAll(async ({ browser }) => {
+  const page = await apiPage(browser)
+  const madeFlow = await api(page, `/flows/${flowName}`, {
+    method: "PUT",
+    data: {
+      name: flowName,
+      title: "Slider drag",
+      version: 1,
+      nodes: [
+        {
+          id: "level",
+          type: "python",
+          title: "Level",
+          requires: [{ name: "level", dtype: "float" }],
+          provides: [{ name: "echo", dtype: "float" }],
+        },
+      ],
+      inputs: [{ spec: { name: "level", dtype: "float" }, initial: 0 }],
+    },
+  })
+  if (!madeFlow.ok())
+    throw new Error(`flow PUT ${madeFlow.status()}: ${await madeFlow.text()}`)
+
+  await api(page, `/flows/${flowName}/nodes/level/source`, {
+    method: "PUT",
+    data: { code: "def process(level=0.0):\n    return {'echo': level}\n" },
+  })
+  // A leftover from a run that failed before its teardown would answer the
+  // create with a version conflict.
+  await api(page, `/dashboards/${dashboard}`, { method: "DELETE" })
+  const madeBoard = await api(page, `/dashboards/${dashboard}`, {
+    method: "PUT",
+    data: {
+      name: dashboard,
+      title: "Slider drag",
+      version: 0,
+      widgets: [
+        {
+          id: "level",
+          type: "slider",
+          title: "Level",
+          layout: { lg: { x: 0, y: 0, w: 6, h: 3 } },
+          config: {
+            target: `${flowName}.level`,
+            dtype: "float",
+            min: 0,
+            max: 100,
+            step: 1,
+          },
+        },
+      ],
+    },
+  })
+  if (!madeBoard.ok())
+    throw new Error(
+      `dashboard PUT ${madeBoard.status()}: ${await madeBoard.text()}`,
+    )
+  const board = await madeBoard.json()
+  const shown = await api(page, `/dashboards/${dashboard}/publish`, {
+    method: "POST",
+    data: { version: board.version },
+  })
+  if (!shown.ok())
+    throw new Error(`publish ${shown.status()}: ${await shown.text()}`)
+  const flow = await (await api(page, `/flows/${flowName}?draft=true`)).json()
+  await api(page, `/flows/${flowName}/publish`, {
+    method: "POST",
+    data: { version: flow.definition.version },
+  })
+  await api(page, `/flows/${flowName}/start`, { method: "POST" })
+  await page.close()
+})
+
+test.afterAll(async ({ browser }) => {
+  await deleteAll(browser, [`/dashboards/${dashboard}`, `/flows/${flowName}`])
+})
+
+test("a dragged slider publishes where it was let go", async ({ page }) => {
+  await page.goto(`/view/${dashboard}`)
+  const slider = page.getByRole("slider", { name: "Level" })
+  await slider.waitFor({ timeout: 20000 })
+
+  const box = (await slider.boundingBox())!
+  const y = box.y + box.height / 2
+
+  // A touch drag across the track, ending on the far side — the gesture a
+  // browser is most willing to mistake for a pan.
+  await page.touchscreen.tap(box.x + 4, y)
+  await page.mouse.move(box.x + 4, y)
+  await page.mouse.down()
+  for (let step = 1; step <= 8; step++) {
+    await page.mouse.move(box.x + (box.width * step) / 10, y, { steps: 2 })
+  }
+  await page.mouse.up()
+
+  // What the flow echoed back is what actually reached the engine.
+  await expect
+    .poll(
+      async () => {
+        const rows = await (await api(page, "/messages/")).json()
+        const echo = (rows.data ?? rows).find(
+          (m: { name: string }) => m.name === `${flowName}.echo`,
+        )
+        return echo?.value ?? 0
+      },
+      { timeout: 15000, message: "the drag never reached the engine" },
+    )
+    .toBeGreaterThan(50)
+
+  await page.mouse.up()
+})
+
+test("the slider owns its own drag gesture", async ({ page }) => {
+  await page.goto(`/view/${dashboard}`)
+  const slider = page.getByRole("slider", { name: "Level" })
+  await slider.waitFor({ timeout: 20000 })
+
+  // The cure for the reported fault, asserted where it can be seen. Without
+  // it a browser is free to read a sideways swipe on the track as a pan or a
+  // back-navigation, take the pointer over mid-drag and end the drag in
+  // `pointercancel` — the handle followed the finger and nothing was ever
+  // published. A drag *is* this control; no gesture outranks it.
+  await expect(slider).toHaveCSS("touch-action", "none")
+})