Chat composer is displaced upward after interacting with a fullscreen MCP App and sending a message

[Dejulia489]

Summary

In Claude Desktop, clicking a control inside a fullscreen MCP App leaves the
conversation in a state where every subsequent message moves the chat composer up away
from the bottom of the window. The displacement accumulates and can carry the composer
out of the viewport.

The behavior is host-side, and it reproduces with the debug-server example in this
repository, copied without modification. Filed here because the
getting-started page
points to this repository for MCP Apps issues.

Three measured properties:

• Until the app is clicked, messages do not displace the composer at all, at any scroll
  depth.
• After the app is clicked, each message adds displacement equal to about half the
  scroll height that message adds.
• The click itself displaces nothing, and returning focus to the composer does not undo
  the state.

Leaving fullscreen, or switching to another conversation, restores the composer.

Environment

• Claude Desktop 1.52386.6.0 (Microsoft Store package) on Windows 11
• Reproduced with examples/debug-server, copied without modification and packaged as a
  local .mcpb desktop extension

Steps to reproduce

1. Package examples/debug-server as a .mcpb and install it in Claude Desktop.
2. In a new conversation, call its tool and put the app in fullscreen display mode.
3. Send two or three messages without clicking anything inside the app. The composer
   stays at the bottom of the window.
4. Click any control inside the app. The composer does not move yet.
5. Send another message. The composer moves up.
6. Send further messages. The composer moves up again each time.

Measurements

Taken in the Claude Desktop DevTools console (Help > Troubleshooting > Enable Developer
Mode, then Ctrl+Shift+I), with the unmodified debug-server app fullscreen:

(() => {
  const c = document.querySelector('[data-chat-input-container]');
  const s = c?.closest('[data-autoscroll-container]');
  return {
    composerGap: Math.round(innerHeight - c.getBoundingClientRect().bottom),
    scrollTop: Math.round(s?.scrollTop ?? -1),
    scrollHeight: Math.round(s?.scrollHeight ?? -1),
    clientHeight: Math.round(s?.clientHeight ?? -1),
  };
})()

composerGap is the distance from the bottom of the window to the bottom of the
composer. When the composer is correctly placed, it is 0.

Sample	composerGap	scrollTop	scrollHeight	clientHeight
Messages sent, app untouched	0	837	2735	1522
Messages sent, app untouched	0	2580	4103	1522
Messages sent, app untouched	0	2984	4507	1522
After clicking in the app, then sending	941	4278	5912	1522

The first three samples show that scroll depth alone does not displace the composer:
scrollTop more than triples while composerGap stays at 0. The fourth sample differs
from them by one action, a click inside the app's iframe.

The accumulation, sampled every 500ms

Once the app has been clicked, a timer logging composerGap, scrollTop and
document.activeElement through two further sends gives this sequence, condensed to the
points where a value changes:

Event	composerGap	scrollTop	activeElement
Start of recording, app already clicked once	1193	4666	DIV[chat-input]
Click inside the app, held for about 10s	1193	4666	IFRAME
Focus returns to the composer	1193	4666	DIV[chat-input]
Send a message	1380	5041	DIV[chat-input]
Send a message	1569	5418	DIV[chat-input]

Two things follow from that sequence.

The displacement advances only when a message is sent, not when the app is clicked.
composerGap is unchanged across the whole period when activeElement is the app's
iframe, and both increases happen while focus is on the composer.

Each increase is close to half the scroll height the message adds. The first send adds
375 to scrollTop and 187 to composerGap, and the second adds 377 and 189.

Additional observations from a second app

A separate MCP App, which presents many controls and so is clicked on constantly, shows
the displacement throughout a fullscreen session. In that app composerGap grew to
3828 in a long conversation, and in a new conversation it tracked the scroller's
scrollTop closely, measuring 281 against a scrollTop of 273.

In every sample from both apps the composer reports position: sticky, so the
displacement is not the sticky positioning being dropped. The app container reports
position: fixed, and dismissing the app changed scrollHeight by 61px, so the app's
frame is not contributing to the conversation scroller's content height.

Impact

A fullscreen MCP App becomes hard to use alongside the conversation, because any use of
the app displaces the composer and every subsequent message displaces it further. An app
with controls cannot avoid the trigger.

Leaving and re-entering fullscreen restores the composer temporarily until the next message is sent, as does switching
conversations.