fix(ui): remove phantom context menu scrollbars

Keep hidden submenus in viewport-fixed positioning from their first layout so they do not enlarge the root context menu scroll area before hover. Preserve viewport-safe submenu placement for mouse and keyboard activation.

src/renderer/styles.css

@@ -243,7 +243,7 @@ body,
 .menu-submenu-dropdown {
   position: absolute;
   top: -4px;
-  left: 100%;
+  left: 0;
   z-index: 1001;
   min-width: max-content;
   background: var(--card);
@@ -2712,7 +2712,7 @@ td {
 
 .ctx-menu-sub-items {
   display: block;
-  position: absolute;
+  position: fixed;
   left: 100%;
   top: 0;
   min-width: 120px;

tests/context-menu.test.tsx

@@ -112,4 +112,9 @@ describe("ContextMenu", () => {
     expect(stylesSource).toMatch(/\.ctx-menu-sub:hover\s*>\s*\.ctx-menu-sub-items\.is-positioned/s);
     expect(contextMenuSource).toContain('window.addEventListener("pointerdown", onOutside, true)');
   });
+
+  it("keeps hidden submenus outside the root menu scroll area before hover", () => {
+    expect(stylesSource).toMatch(/\.ctx-menu-sub-items\s*{[^}]*position:\s*fixed;/s);
+    expect(stylesSource).not.toMatch(/\.ctx-menu-sub-items\s*{[^}]*position:\s*absolute;/s);
+  });
 });