Flip prompt popover above the cursor when a scroll container would clip it

[shved270189]

Problem

The prompt/mention popover only flips above the cursor when it would overflow the window (popoverRect.bottom > window.innerHeight). When a <lexxy-editor> lives inside a container that clips overflow — e.g. a fixed-height scroll area or modal composer — the menu opens downward and is cut off by that container even though the window still has room below it.

Fix

Flip the menu above the cursor when the popover would overflow the editor's nearest clipping ancestor (an ancestor whose computed overflow-y is not visible), capped by the window — instead of checking the window alone:

#availableBottom() {
  let bottom = window.innerHeight
  let node = this.#editorElement

  while (node && node !== document.body && node !== document.documentElement) {
    if (getComputedStyle(node).overflowY !== "visible") {
      bottom = Math.min(bottom, node.getBoundingClientRect().bottom)
    }
    node = node.parentElement
  }

  return bottom
}

When the editor has no clipping ancestor the bound is window.innerHeight, so an ordinary editor keeps the menu below the cursor exactly as before — the flip only kicks in when something would actually clip it.

Tests

New test/browser/tests/prompts/overflow_container.test.js + fixture, covering both directions:

• editor inside a fixed-height scroll container with the window still having room below → menu flips above the cursor and stays within the container;
• the same editor with no clipping ancestor → menu stays below the cursor (guards against flipping when nothing clips).

Both new tests pass on chromium, firefox, and webkit; the full prompts suite passes on chromium.

[claude]

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