Fix cursor jumping when moving in whitespace at EOL

This fixes the issue with the cursor jumping, but does not fix
the underlying problem, which is related to us not providing a
good API for including the whitespace in text width calculations.

see #1430

druid/src/widget/textbox.rs

@@ -227,7 +227,10 @@ impl<T: TextStorage + EditableText> TextBox<T> {
     /// Calculate a stateful scroll offset
     fn update_hscroll(&mut self, self_width: f64, env: &Env) {
         let cursor_x = self.editor.cursor_line().p0.x;
-        let overall_text_width = self.editor.layout().size().width;
+        // if the text ends in trailing whitespace, that space is not included
+        // in its reported width, but we need to include it for these calculations.
+        // see https://github.com/linebender/druid/issues/1430
+        let overall_text_width = self.editor.layout().size().width.max(cursor_x);
         let text_insets = env.get(theme::TEXTBOX_INSETS);
 
         //// when advancing the cursor, we want some additional padding