fix(app): stop the composer dropzone flickering and showing for text drags - #60746

Summary

• Regression from: feat(session-ui): rewrite v2 prompt input #37102 (v2 prompt input rewrite). It mapped every drag type to one drag.enter state, so the text drags that feat(app): drag-n-drop to @mention file #12569 treated as @mention drags now opened the file dropzone. It also added the editor dragleave handler that clears the state.
• Text drags showed the file dropzone. The document dragover listener treated any drag that carried text/plain as a file drag. When you dragged a selection in the composer, "Drop images, PDFs, or files to add" covered the page.
• File drags flickered over the composer. The editor's dragleave cleared the drag state every time the pointer moved between the editor's own nodes. The next dragover set it again, so the dropzone unmounted, remounted, and restarted its fade on each move.
• Fix: The document dragover owns the drag state. The editor no longer sets or clears it. The dropzone opens only for drags that carry Files (from the system) or text/uri-list (file rows from the workspace tree).

Before

After

sequenceDiagram
  participant D as document
  participant E as editor
  participant S as drag state
  Note over D,S: Before: pointer moves from the editor onto its text
  E->>S: dragleave G�� idle (dropzone unmounts)
  D->>S: dragover G�� active (dropzone remounts, fade restarts)
  Note over D,S: After
  D->>S: dragover G�� active (no change)

Implementation

• attachments.ts: dragover sets the state from the drag's types:

  function dragType(data: DataTransfer | null) {
    if (data?.types.includes("Files")) return "image"
  
    if (data?.types.includes("text/uri-list")) return "@mention"
  
    return null
  }

• interaction.ts / editor.tsx: The editor's dragenter only calls preventDefault(), and it has no dragleave handler. onDragLeave is now clearDrag. The session controller calls it when the composer becomes hidden.

• composer.spec.ts: A new keeper checks two things. A text drag shows no dropzone. When a file drag moves onto the editor's text, the dropzone element stays mounted and visible.

Visual

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  <g opacity=".12">
    <path d="M40 60 L64 12 M64 60 L88 12 M88 60 L112 12 M112 60 L136 12 M136 60 L160 12 M160 60 L184 12 M184 60 L208 12" stroke="currentColor" stroke-width="6"/>
  </g>
  <circle cx="120" cy="24" r="8" opacity=".25"/>
  <path d="M120 20 L120 28 M116 23 L120 19 L124 23" stroke="currentColor" stroke-width="1.6" fill="none" opacity=".8"/>
  <rect x="84" y="38" width="72" height="6" rx="3" opacity=".7"/>
  <rect x="44" y="50" width="152" height="12" rx="4" opacity=".14"/>
  <g>
    <animateTransform attributeName="transform" type="translate" values="0 0;100 0;0 0" dur="3s" repeatCount="indefinite" calcMode="spline" keySplines=".4 0 .2 1;.4 0 .2 1"/>
    <rect x="52" y="46" width="14" height="18" rx="2" opacity=".55"/>
    <path d="M62 54 l5 5 l-2 0 l1.5 3 l-1.5 0.7 l-1.5 -3 l-1.5 1.5 z" opacity=".9"/>
  </g>
</svg>