mcp: be smart about delegating scroll in mcp apps

[connor4312]

Implements smart scroll delegation for MCP app iframes. Instead of
delegating 100% of wheel events when content is below max height (or 0%
when scrollable), now detects scroll boundaries and only bubbles events
when the iframe's content is scrolled to the top (scrolling up) or bottom
(scrolling down).

• Adds scroll boundary detection in the MCP app preamble script that
  checks both element-level scrolling (overflow: auto/scroll) and
  document-level scrolling (overflow: visible on html/body)
• Introduces ui/notifications/sandbox-wheel notification to signal when
  scroll events should bubble from the iframe to the chat widget
• Adds CustomSandboxWheelNotification interface to McpApps protocol
• Removes the old canScrollWithin derived observable that made blanket
  delegation decisions based only on content height

(Commit message generated by Copilot)

[vs-code-engineering]

📬 CODENOTIFY

The following users are being notified based on files changed in this PR:

@bpasero

Matched files:

• src/vs/workbench/contrib/chat/browser/agentSessions/agentSessionHoverWidget.ts
• src/vs/workbench/contrib/chat/browser/agentSessions/media/agentSessionHoverWidget.css

[Copilot]

Pull request overview

This PR implements smart scroll delegation for MCP app iframes by detecting scroll boundaries instead of using a blanket delegation strategy based on content height. The changes enable wheel events to bubble from the iframe to the chat widget only when the iframe's content is scrolled to its boundaries.

Changes:

• Adds scroll boundary detection logic to the MCP app preamble script that checks both element-level and document-level scrolling
• Introduces a new CustomSandboxWheelNotification protocol message to communicate wheel events from the iframe to the host
• Replaces the old canScrollWithin observable-based approach with event-driven boundary detection
• Includes unrelated UI improvements to the agent session hover widget (adding provider names)

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
src/vs/workbench/contrib/mcp/common/modelContextProtocolApps.ts	Adds CustomSandboxWheelNotification interface to the McpApps protocol
src/vs/workbench/contrib/chat/browser/widget/chatContentParts/toolInvocationParts/chatMcpAppModel.ts	Removes old scroll delegation logic, adds scroll boundary detection in preamble script, implements _handleSandboxWheel handler
src/vs/workbench/contrib/chat/browser/agentSessions/agentSessionHoverWidget.ts	Adds provider names to agent session hover widget (unrelated to scroll delegation)
src/vs/workbench/contrib/chat/browser/agentSessions/media/agentSessionHoverWidget.css	Adds CSS styling for codicon display (unrelated to scroll delegation)

Comments suppressed due to low confidence (1)

src/vs/workbench/contrib/chat/browser/widget/chatContentParts/toolInvocationParts/chatMcpAppModel.ts:663

• The deltaY value is being negated when constructing the mock IMouseWheelEvent (line 663), but StandardWheelEvent's constructor (which is called by delegateScrollFromMouseWheelEvent) will negate deltaY again when processing the wheel event (see mouseEvent.ts line 181). This double negation will cause scrolling to occur in the wrong direction. The params.deltaY value should be passed through without negation, as StandardWheelEvent handles the sign conversion internally.