feat(desktop): move the Project Canvas beside project chat

The Canvas preview sat above the message timeline as a fixed 25.2/28.8rem
strip, so it ate the top of every project channel's chat: the timeline
started below the fold, and a "Chat preview boundary" marker had to be
drawn across the full Canvas to show authors where that fold landed.

Put the preview in a right-hand pane instead. Chat keeps its full height,
and the Canvas gets a tall column, which suits a dashboard of stacked
widgets far better than a letterbox.

- `ChannelViewOverride` gains `mainColumnHeaderPlacement`
  ("top" | "right", default top). `ChannelPaneMainColumn` lays the header
  out as a sibling column (`order-last`, `clamp(24rem, 38%, 28.8rem)`)
  when it is "right" and keeps the existing `contents` pass-through
  otherwise, so no other channel view changes. `ProjectChannelHome` asks
  for "right" on the chat tab and "top" on the canvas tab, where the
  surface still fills the pane.
- The pane is container-queried, not viewport-queried: it appears only
  once the chat column itself is at least 52rem wide, so a narrow window
  shows chat at full width and users reach the Canvas through its tab.
  `ChannelPane` also passes `hideRightHeader` when a split auxiliary pane
  is open, so a thread, profile, or agent panel takes that space rather
  than competing with it. The frame stays mounted (`display: none`), so
  opening and closing a thread never reloads the package or drops its
  live subscriptions.
- The preview-boundary marker goes away with the fold it described, and
  "Show full canvas" becomes a tooltipped icon button pinned
  bottom-right, since a full-width labelled button no longer fits a 24rem
  column. It follows the existing icon-button idiom: one `aria-label` on
  the button, matching tooltip copy.

The e2e spec binds the new geometry — chat and composer both end at or
before the pane's left edge and the pane is taller than it is wide; the
narrow-viewport test now asserts the pane is hidden and the full tab is
the way in; and a new 1920px test opens a thread to check the pane
yields, chat takes the width, and the frame's `src` survives the round
trip.

Verified: desktop unit suite 5987 passing, project-channel-canvas 8
passing, the project and thread layout specs 6 passing, and tsc, biome,
px-text, pubkey-truncation, and the file-size gate clean. Three specs
fail on this branch independent of this change —
project-cold-start.spec.ts:76 and :107 plus
project-empty-state-alignment.spec.ts:48 look for
`project-home-context-panel`, whose component has had no callers since
the channel-first migration.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Signed-off-by: Matt Toohey <contact@matttoohey.com>

desktop/src/features/channels/ui/ChannelPane.tsx

@@ -623,7 +623,7 @@ export const ChannelPane = React.memo(function ChannelPane({
               <ThreadRepliesErrorCard onRetry={onRetryHuddleThreadReplies} />
             </div>
           ) : null}
-          <ChannelPaneMainColumn>
+          <ChannelPaneMainColumn hideRightHeader={hasSplitAuxiliaryPane}>
             <MessageTimeline
               ref={messageTimelineRef}
               channelId={activeChannel?.id}

desktop/src/features/channels/ui/ChannelPaneMainColumn.tsx

@@ -11,11 +11,16 @@ const IN_FLOW_CHANNEL_CONTENT_STYLE = {
 
 export function ChannelPaneMainColumn({
   children,
+  hideRightHeader = false,
 }: {
   children: React.ReactNode;
+  hideRightHeader?: boolean;
 }) {
   const channelView = useChannelViewOverride();
   const mainColumnHeader = channelView?.mainColumnHeader;
+  const headerOnRight =
+    Boolean(mainColumnHeader) &&
+    channelView?.mainColumnHeaderPlacement === "right";
   const className = cn(
     "relative isolate flex min-h-0 min-w-0 flex-1 flex-col",
     channelView?.mainContent && "hidden",
@@ -32,12 +37,37 @@ export function ChannelPaneMainColumn({
         )}
       >
         <div
-          className="relative flex min-h-0 min-w-0 flex-1 flex-col"
+          className={cn(
+            "relative flex min-h-0 min-w-0 flex-1",
+            headerOnRight
+              ? "flex-row [container-type:inline-size]"
+              : "flex-col",
+          )}
           style={IN_FLOW_CHANNEL_CONTENT_STYLE}
         >
-          {mainColumnHeader}
           <div
-            className={channelView?.hideMainColumnBody ? "hidden" : "contents"}
+            className={cn(
+              headerOnRight
+                ? cn(
+                    "order-last min-h-0 shrink-0 [width:clamp(24rem,38%,28.8rem)]",
+                    hideRightHeader
+                      ? "hidden"
+                      : "hidden [@container(min-width:52rem)]:flex",
+                  )
+                : "contents",
+            )}
+            data-testid="channel-main-column-header"
+          >
+            {mainColumnHeader}
+          </div>
+          <div
+            className={cn(
+              channelView?.hideMainColumnBody
+                ? "hidden"
+                : headerOnRight
+                  ? "relative flex min-h-0 min-w-0 flex-1 flex-col"
+                  : "contents",
+            )}
             data-testid="channel-main-column-body"
           >
             {children}

desktop/src/features/channels/ui/ChannelViewOverrideContext.tsx

@@ -5,6 +5,7 @@ type ChannelViewOverride = {
   hideMainColumnBody?: boolean;
   isChannelViewActive: boolean;
   mainColumnHeader?: React.ReactNode;
+  mainColumnHeaderPlacement?: "top" | "right";
   mainContent: React.ReactNode;
   onSelectChannelView: () => void;
 };

desktop/src/features/projects/ui/ProjectChannelHome.tsx

@@ -700,6 +700,8 @@ export function ProjectChannelHome({
                             snapshots={canvasSnapshots}
                           />
                         ) : null,
+                      mainColumnHeaderPlacement:
+                        activeView === "chat" ? "right" : "top",
                       mainContent,
                       onSelectChannelView: () => selectView("chat"),
                     }}

desktop/src/features/projects/ui/project-canvas/ProjectCanvasSurface.tsx

@@ -2,6 +2,7 @@ import { Maximize2 } from "lucide-react";
 
 import { cn } from "@/shared/lib/cn";
 import { Button } from "@/shared/ui/button";
+import { Tooltip, TooltipContent, TooltipTrigger } from "@/shared/ui/tooltip";
 import { ProjectCanvasHost } from "./ProjectCanvasHost";
 import type { ProjectCanvasBroker } from "./projectCanvasBroker";
 import type { ProjectCanvasSnapshots } from "./projectCanvasProtocol";
@@ -32,9 +33,7 @@ export function ProjectCanvasSurface({
     <div
       className={cn(
         "relative flex min-h-0 flex-col overflow-hidden bg-background",
-        full
-          ? "flex-1"
-          : "h-[25.2rem] shrink-0 border-b border-border md:h-[28.8rem]",
+        full ? "flex-1" : "h-full w-full border-l border-border",
       )}
       data-canvas-mode={full ? "full" : "preview"}
       data-testid="project-canvas-surface"
@@ -60,32 +59,24 @@ export function ProjectCanvasSurface({
           snapshots={snapshots}
         />
       </div>
-      {full ? (
-        <div
-          aria-hidden="true"
-          className="pointer-events-none absolute inset-x-0 top-[25.2rem] z-30 flex items-center gap-3 px-3 md:top-[28.8rem]"
-          data-testid="project-canvas-preview-boundary"
-        >
-          <div className="h-px flex-1 border-t border-dotted border-primary/65" />
-          <span className="rounded-sm bg-background/90 px-2 py-0.5 text-3xs font-medium text-muted-foreground shadow-sm backdrop-blur-sm">
-            Chat preview boundary
-          </span>
-          <div className="h-px flex-1 border-t border-dotted border-primary/65" />
-        </div>
-      ) : null}
       {!full ? (
-        <div className="absolute inset-x-0 bottom-3 z-30 flex justify-center px-4">
-          <Button
-            className="border-border/80 bg-background/95 shadow-sm backdrop-blur-sm"
-            data-testid="project-canvas-show-full"
-            onClick={onShowFullCanvas}
-            size="sm"
-            type="button"
-            variant="outline"
-          >
-            <Maximize2 className="h-4 w-4" />
-            Show full canvas
-          </Button>
+        <div className="absolute bottom-3 right-3 z-50">
+          <Tooltip disableHoverableContent>
+            <TooltipTrigger asChild>
+              <Button
+                aria-label="Show full Canvas"
+                className="h-8 w-8 border-border/80 bg-background/95 shadow-sm backdrop-blur-sm"
+                data-testid="project-canvas-show-full"
+                onClick={onShowFullCanvas}
+                size="icon"
+                type="button"
+                variant="outline"
+              >
+                <Maximize2 className="h-4 w-4" />
+              </Button>
+            </TooltipTrigger>
+            <TooltipContent>Show full Canvas</TooltipContent>
+          </Tooltip>
         </div>
       ) : null}
     </div>

desktop/tests/e2e/project-channel-canvas.spec.ts

@@ -1,6 +1,6 @@
 import { expect, test, type Page } from "@playwright/test";
 
-import { installMockBridge } from "../helpers/bridge";
+import { installMockBridge, TEST_IDENTITIES } from "../helpers/bridge";
 
 async function openStarterProject(page: Page) {
   const projectRow = page.getByTestId("sidebar-project-buzz");
@@ -12,6 +12,20 @@ async function openStarterProject(page: Page) {
   await projectRow.click();
 }
 
+async function waitForMockLiveSubscription(page: Page, channelName: string) {
+  await expect
+    .poll(() =>
+      page.evaluate(
+        (name) =>
+          window.__BUZZ_E2E_HAS_MOCK_LIVE_SUBSCRIPTION__?.({
+            channelName: name,
+          }) ?? false,
+        channelName,
+      ),
+    )
+    .toBe(true);
+}
+
 async function expectCanvasReady(page: Page) {
   const iframe = page.getByTestId("project-canvas-frame");
   await expect(iframe).toHaveAttribute("sandbox", "allow-scripts");
@@ -26,7 +40,7 @@ async function expectCanvasReady(page: Page) {
   return { iframe, root };
 }
 
-test("project canvas uses one sandboxed frame across preview and full modes", async ({
+test("project canvas uses one sandboxed frame across right-pane and full modes", async ({
   page,
 }) => {
   await installMockBridge(page);
@@ -40,7 +54,22 @@ test("project canvas uses one sandboxed frame across preview and full modes", as
   const surface = page.getByTestId("project-canvas-surface");
   await expect(surface).toHaveAttribute("data-canvas-mode", "preview");
   await expect(page.getByTestId("project-canvas-show-full")).toBeVisible();
-  await expect(page.getByTestId("channel-composer-overlay")).toBeVisible();
+  const composer = page.getByTestId("channel-composer-overlay");
+  await expect(composer).toBeVisible();
+
+  const bodyBox = await page
+    .getByTestId("channel-main-column-body")
+    .boundingBox();
+  const composerBox = await composer.boundingBox();
+  const surfaceBox = await surface.boundingBox();
+  if (!bodyBox || !composerBox || !surfaceBox) {
+    throw new Error("Project chat and Canvas pane were not visible.");
+  }
+  expect(surfaceBox.height).toBeGreaterThan(surfaceBox.width);
+  expect(bodyBox.x + bodyBox.width).toBeLessThanOrEqual(surfaceBox.x + 1);
+  expect(composerBox.x + composerBox.width).toBeLessThanOrEqual(
+    surfaceBox.x + 1,
+  );
 
   const { iframe, root } = await expectCanvasReady(page);
   const initialSource = await iframe.getAttribute("src");
@@ -87,9 +116,9 @@ test("project canvas uses one sandboxed frame across preview and full modes", as
     "aria-selected",
     "true",
   );
-  await expect(
-    page.getByTestId("project-canvas-preview-boundary"),
-  ).toBeVisible();
+  await expect(page.getByTestId("project-canvas-preview-boundary")).toHaveCount(
+    0,
+  );
   await expect(page.getByTestId("channel-composer-overlay")).toBeHidden();
   await expect(iframe).toHaveAttribute("src", initialSource ?? "");
   await expect(root).toHaveAttribute("data-canvas-mode", "full");
@@ -140,6 +169,47 @@ test("project canvas uses one sandboxed frame across preview and full modes", as
     .toBe(2);
 });
 
+test("project canvas yields to a right-side panel at a wide viewport", async ({
+  page,
+}) => {
+  await page.setViewportSize({ height: 900, width: 1920 });
+  await installMockBridge(page);
+  await page.goto("/");
+  await openStarterProject(page);
+  await expect(page.getByTestId("project-channel-home")).toBeVisible();
+  await waitForMockLiveSubscription(page, "buzz");
+
+  const surface = page.getByTestId("project-canvas-surface");
+  await expect(surface).toBeVisible();
+  const { iframe } = await expectCanvasReady(page);
+  const initialSource = await iframe.getAttribute("src");
+  const rootId = await page.evaluate((pubkey) => {
+    return (
+      window.__BUZZ_E2E_EMIT_MOCK_MESSAGE__?.({
+        channelName: "buzz",
+        content: "Project thread that opens beside chat.",
+        pubkey,
+      })?.id ?? null
+    );
+  }, TEST_IDENTITIES.tyler.pubkey);
+  expect(rootId).not.toBeNull();
+
+  await page.getByTestId(`reply-message-${rootId}`).click({ force: true });
+  await expect(page.getByTestId("message-thread-panel")).toBeVisible();
+  await expect(surface).toBeHidden();
+  const bodyBox = await page
+    .getByTestId("channel-main-column-body")
+    .boundingBox();
+  if (!bodyBox) throw new Error("Project chat was not visible beside thread.");
+  expect(bodyBox.width).toBeGreaterThan(832);
+  await expect(iframe).toHaveAttribute("src", initialSource ?? "");
+
+  await page.getByTestId("auxiliary-panel-close").click();
+  await expect(page.getByTestId("message-thread-panel")).toHaveCount(0);
+  await expect(surface).toBeVisible();
+  await expect(iframe).toHaveAttribute("src", initialSource ?? "");
+});
+
 test("Reload activates a new package revision and releases the old handle", async ({
   page,
 }) => {
@@ -252,7 +322,7 @@ test("a failed candidate commit restores the active Canvas", async ({
     .toBe(2);
 });
 
-test("project canvas preview and full tab stay contained at a narrow viewport", async ({
+test("project canvas uses the full tab instead of squeezing narrow chat", async ({
   page,
 }) => {
   await page.setViewportSize({ height: 844, width: 390 });
@@ -263,26 +333,26 @@ test("project canvas preview and full tab stay contained at a narrow viewport",
   await page.keyboard.press("Escape");
 
   const surface = page.getByTestId("project-canvas-surface");
-  const { iframe } = await expectCanvasReady(page);
   await expect(surface).toHaveAttribute("data-canvas-mode", "preview");
-  await expect(page.getByTestId("project-canvas-show-full")).toBeVisible();
+  await expect(surface).toBeHidden();
+  await expect(page.getByTestId("project-canvas-show-full")).toBeHidden();
+  await expect(page.getByTestId("channel-composer-overlay")).toBeVisible();
+
+  await page.getByTestId("project-channel-tab-canvas").click();
+  await expect(surface).toHaveAttribute("data-canvas-mode", "full");
+  await expect(surface).toBeVisible();
+  await expect(page.getByTestId("channel-composer-overlay")).toBeHidden();
+  const { iframe, root } = await expectCanvasReady(page);
+  await expect(root).toHaveAttribute("data-canvas-mode", "full");
   const surfaceBox = await surface.boundingBox();
   const frameBox = await iframe.boundingBox();
-  if (!surfaceBox || !frameBox)
-    throw new Error("Canvas frame was not visible.");
+  if (!surfaceBox || !frameBox) {
+    throw new Error("Full Canvas frame was not visible.");
+  }
   expect(frameBox.x).toBeGreaterThanOrEqual(surfaceBox.x);
   expect(frameBox.x + frameBox.width).toBeLessThanOrEqual(
     surfaceBox.x + surfaceBox.width,
   );
-
-  await page.getByTestId("project-canvas-show-full").click();
-  await expect(surface).toHaveAttribute("data-canvas-mode", "full");
-  await expect(page.getByTestId("channel-composer-overlay")).toBeHidden();
-  await expect(
-    page
-      .frameLocator('[data-testid="project-canvas-frame"]')
-      .locator("#canvas-root"),
-  ).toHaveAttribute("data-canvas-mode", "full");
 });
 
 test("unexpected child navigation tears down the Canvas frame", async ({