Set overflow scroll to auto for mirror div

Align overflow scroll for the mirror div with the textarea. Otherwise if the
textarea did not have scrollbars, text in the mirror div would wrap sooner than
text in the textarea.

On macOS where overlay scrollbars are used this has no effect, since the layout
of the text is unaffected by whether scrollbars are "present".

Author: robertknight

src/sidebar/util/textarea-caret-position.ts

@@ -72,7 +72,7 @@ export function getCaretCoordinates(textarea: HTMLTextAreaElement): {
     // Adjust scroll position to match the textarea. For this to work, the
     // scrollable height of the mirror must be at least equal to the scrollable
     // height of the text area.
-    mirrorDiv.style.overflow = 'scroll';
+    mirrorDiv.style.overflowY = 'auto';
     mirrorDiv.scrollTop = textarea.scrollTop;
 
     // Capture caret position relative to top-left corner of textarea.