xterm runs the custom key handler for keypress too and drops Ctrl/Alt
keypresses but not Shift-only ones, so the stray \r submitted the prompt
after the newline. Swallow every event type for Shift/Ctrl+Enter and send
only on keydown, in the primary pane and Pane B. Adds a static guard and a
real xterm + Chromium browser test.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
