fix(web): closing a pane no longer blacks out the panes beside it

apps/web/src/layout/SurfaceHost.tsx

@@ -185,12 +185,16 @@ const NO_GHOSTS: ReadonlyMap<string, ClosingTab> = new Map();
  * Pane edges land on whole pixels, so borders and text stay crisp at any split.
  * Both panes round the same boundary, so the gap between them stays exact.
  */
-function slotStyle(slot: LayoutPaneSlot): React.CSSProperties {
+function slotStyle(slot: LayoutPaneSlot, stripPx?: number): React.CSSProperties {
   const edge = (has: boolean) => (has ? HALF_PANE_GAP : "0px");
   const at = (fraction: number, offset: string) =>
     `round(calc(${fraction * 100}% + ${offset}), 1px)`;
-  const left = at(slot.rect.left, edge(slot.hasLeft));
-  const right = at(slot.rect.left + slot.rect.width, `-1 * ${edge(slot.hasRight)}`);
+  const across = (fraction: number, offset: string) =>
+    stripPx === undefined
+      ? at(fraction, offset)
+      : `round(calc(${fraction * stripPx}px + ${offset}), 1px)`;
+  const left = across(slot.rect.left, edge(slot.hasLeft));
+  const right = across(slot.rect.left + slot.rect.width, `-1 * ${edge(slot.hasRight)}`);
   const top = at(slot.rect.top, edge(slot.hasAbove));
   const bottom = at(slot.rect.top + slot.rect.height, `-1 * ${edge(slot.hasBelow)}`);
   return {
@@ -1056,6 +1060,7 @@ export function LayoutSurfaceHost({
     chats: chatTabsById,
     launchers: launcherTabsById,
     placed: slotByTabId,
+    stripPx,
     motionKey: paneMotionKey,
     framed: paneCount > 1 || maximizedTabId !== null,
   });
@@ -1068,6 +1073,7 @@ export function LayoutSurfaceHost({
       chats: chatTabsById,
       launchers: launcherTabsById,
       placed: slotByTabId,
+      stripPx,
       motionKey: paneMotionKey,
       framed: paneCount > 1 || maximizedTabId !== null,
     });
@@ -1080,8 +1086,9 @@ export function LayoutSurfaceHost({
       seen.motionKey === paneMotionKey
     ) {
       const closed: [string, ClosingTab][] = [];
-      for (const [tabId, placed] of seen.placed) {
+      for (const [tabId, was] of seen.placed) {
         if (presentTabIds.has(tabId) || ghosts.has(tabId)) continue;
+        const placed = { slot: was.slot, style: slotStyle(was.slot, seen.stripPx) };
         const chat = seen.chats.get(tabId);
         const launcher = seen.launchers.get(tabId);
         if (chat?.tab.content.kind === "chat") {

apps/web/src/motion/paneMotion.browser.test.ts

@@ -0,0 +1,16 @@
+import { assert, describe, it } from "vite-plus/test";
+
+import { findDonor } from "./paneMotion";
+
+const column = (x: number, width: number) => ({ x, y: 0, width, height: 880 });
+
+describe("findDonor", () => {
+  it("takes only a neighbour across the gutter, never one with panes between", () => {
+    const closed = column(872, 416);
+    const far = { box: column(0, 442), other: column(0, 1288) };
+    const beside = { box: column(450, 414), other: column(450, 838) };
+
+    assert.isNull(findDonor(closed, [far]));
+    assert.strictEqual(findDonor(closed, [far, beside])?.neighbour, beside);
+  });
+});

apps/web/src/motion/paneMotion.ts

@@ -38,6 +38,8 @@ const translate = (dx: number, dy: number): Keyframe => ({
 /** How far a new pane slides out from under its donor, in px. */
 const OPEN_TRAVEL_PX = 48;
 
+const ADJACENT_GAP_PX = 16;
+
 /** The Panel animations setting's default, which plays niri's springs as they are. */
 const DEFAULT_PANEL_MS = 150;
 
@@ -123,7 +125,7 @@ interface Neighbour {
  * donor. One that changed size along that side wins; then left, above,
  * right, below, the nearest on each side.
  */
-function findDonor(
+export function findDonor(
   box: Box,
   neighbours: Iterable<Neighbour>,
 ): { neighbour: Neighbour; side: Side; gap: number } | null {
@@ -145,7 +147,7 @@ function findDonor(
       );
     }
     for (const [side, gap] of gaps) {
-      if (gap < -1) continue;
+      if (gap < -1 || gap > ADJACENT_GAP_PX) continue;
       const rank = (candidate: { neighbour: Neighbour; side: Side }) =>
         (candidate.neighbour.other &&
         resized(candidate.side, candidate.neighbour.box, candidate.neighbour.other)