Size desktop-hosted preview tabs to the remote viewer's panel (blurry, letterboxed streams)

[WhiteWarrior625]

Note

Posted by Claude Code (Opus 5.5) on behalf of the account owner. Follow-up to #19420, where the 1280x800 stream was described as current behaviour and an Ideas discussion was suggested.

Problem. A desktop client that views a remote environment's tab gets a JPEG screencast of a hidden 1280x800 DPR-1 <webview> on the host. That tab never follows the viewer's panel:

• Page size. The hidden webview uses the last on-screen rect, or 1280x800 if the tab was never on screen (HostedBrowserWebview.tsx:221-232), at the host display's DPR.
• Resize and viewport input. The viewer's resize is ignored for desktop tabs (ServerBrowser.ts:2201). applySetting and applyZoom return early for desktop tabs (ServerBrowser.ts:609-610 and :632-635).
• Frame cap. The viewer caps frames at its panel size times devicePixelRatio (ServerBrowserSurface.tsx:318-331), sent as maxWidth and maxHeight (:371). The server passes those to Page.startScreencast as JPEG at quality 70, dropping to half scale at quality 50 while scrolling (ServerBrowser.ts:2280-2291, ServerBrowserStream.ts:32 and :89-91).
• Display. The canvas is object-contain (ServerBrowserSurface.tsx:747).

On my Windows client, the panel asked for maxWidth=674&maxHeight=948 (from the host's request log). A 1280x800 page fits that cap as a 674x421 frame: about 0.53 scale, centered, with bands above and below. Text at that scale cannot be read. A headless server tab gets the viewer's size at 2x render scale (RENDER_SCALE, ServerBrowser.ts:88, :2196-2206). A desktop-hosted tab gets neither, and taking control does not change that.

Proposal. While a remote viewer controls a desktop-hosted tab in fill mode, forward that viewer's resize to the host's HostedBrowserWebview as its hidden size. Alternatively, give remote-viewed tabs a never-shown BrowserWindow, like #22743, sized from the viewer at the viewer's DPR. Either change would also give #19456 a size that matches what the viewer sees.

Workaround for users today. Use Open on this computer to move the tab into the client's own browser. That only works when the page is reachable from the client. For loopback-only dev servers the rewritten URL (resolveDiscoveredServerUrl, browserTargetResolver.ts:92-110) points at the host's LAN address and gets connection refused. Agents on the host also cannot drive a tab that has moved to the client.