fix(mobile): end mouse gestures whose pointerup never reached the surface

Capture the mouse pointer on pointerdown so a drag that leaves the surface
keeps delivering pointermove/pointerup; when capture is unavailable and the
release is lost anyway, synthesize the release from the next buttons==0
pointermove or the next pointerdown, so a tracking TUI is never left with
the left button latched down.

mobile/src/terminal/terminal-webview-mouse-click-drag-injected.ts

@@ -128,7 +128,13 @@ export const TERMINAL_MOUSE_CLICK_DRAG_JS = `
     targetSurface.addEventListener('pointermove', function(e) {
       var gesture = mouseGesture;
       if (e.pointerType !== 'mouse' || !gesture || gesture.mode === 'cancelled') return;
-      if (!term || (e.buttons & 1) === 0) return;
+      if (!term) return;
+      if ((e.buttons & 1) === 0) {
+        // Why: a pointerup lost outside the WebView (capture unavailable) must
+        // end the gesture here, or a tracked press stays latched at the TUI.
+        abandonMouseGesture();
+        return;
+      }
       gesture.lastX = e.clientX;
       gesture.lastY = e.clientY;
       if (!gesture.moved) {

mobile/src/terminal/terminal-webview-mouse-click-drag.test.ts

@@ -254,7 +254,7 @@ describe('terminal WebView external mouse click and drag', () => {
     expect(new Set(motionCols).size).toBe(motionCols.length)
   })
 
-  it('does not report press or motion for an x10 click-only TUI drag', () => {
+  it('does not report motion or release for an x10 click-only TUI drag', () => {
     boot()
     activeTerminal().modes.mouseTrackingMode = 'x10'
 
@@ -338,6 +338,26 @@ describe('terminal WebView external mouse click and drag', () => {
     expect(terminalInputBytes(postMessage)).toBe('')
   })
 
+  it('releases a tracked drag when the button state shows the pointerup was lost', () => {
+    boot()
+    activeTerminal().modes.mouseTrackingMode = 'drag'
+
+    dispatchPointer('pointerdown', { x: 40, y: 60, button: 0, buttons: 1 })
+    dispatchPointer('pointermove', { x: 160, y: 60, button: 0, buttons: 1 })
+    // Pointer re-enters with the button already up — the pointerup never arrived.
+    dispatchPointer('pointermove', { x: 200, y: 60, button: 0, buttons: 0 })
+
+    const reports = terminalInputBytes(postMessage).match(DEFAULT_MOUSE_REPORT_RE) ?? []
+    expect(reports[reports.length - 1]?.charCodeAt(3)).toBe(35)
+
+    postMessage.mockClear()
+    dispatchPointer('pointermove', { x: 240, y: 60, button: 0, buttons: 1 })
+
+    // The lost pointerup ended the gesture; a later unrelated move must not
+    // keep emitting motion for it.
+    expect(terminalInputBytes(postMessage)).toBe('')
+  })
+
   it('releases a tracked drag when the pointer is cancelled mid-gesture', () => {
     boot()
     activeTerminal().modes.mouseTrackingMode = 'drag'