The Textbox for entering the URI in the Composer's Parsed tab appears to be a standard Textbox/Edit Control. Pressing Ctrl+Left Arrow or Ctrl+Right Arrow causes the caret to jump to the nearest word boundary which in a URI is not very helpful as it only snaps to the very beginning, the very end, and to the query string separator ("?") by default.
Using EM_SETWORDBREAKPROC would allow an alternate callback function to be provided for determining where the word breaks are in a URL and cause Ctrl+arrow to position the caret at more useful locations for editing the URI.
My initial thoughts are that the path separator ("/") and the query string key-value pair separator ("&") would be good candidates for additional word boundaries. Retain the "?" as a word boundary.