fix: avoid retained webview flash at default iframe size

[cipheraxat]

Summary

• Retained custom-editor webviews (retainContextWhenHidden) could briefly paint at the browser default iframe size (~300×150) when switching back from a text editor, because OverlayWebview._show() set visibility: visible before a connected CSS anchor was bound.
• Defer unhide until a claimed overlay has a connected anchor (_updateVisibility), and bind the editor CSS anchor before claim when the editor element is already connected.
• Fixes Retained custom editor webview briefly renders at default iframe size when switching from a text editor #323890

Test plan

• In-product: minimal retainContextWhenHidden custom editor; open → switch to text tab → switch back; first visible overlay/iframe size matches editor bounds (no ≤320×180 flash across reveal samples)
• Logic: visibility only when owner is set and anchorElement.isConnected
• Manual: non-retained webviews still show normally
• Manual: webview views / extension editor that claim without re-anchoring still appear when the prior anchor stays connected

[Copilot]

Pull request overview

Prevents retained custom-editor webviews from briefly rendering at the iframe’s default size during reveal.

Changes:

• Defers overlay visibility until ownership and a connected CSS anchor are established.
• Binds editor anchors before claiming retained webviews.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
webviewEditor.ts	Anchors webviews before claiming them.
overlayWebview.ts	Gates visibility on ownership and anchor connectivity.