[LyX/master] Fixup 4671214f: add comment

[Jean-Marc Lasgouttes]

commit 0429b580e6f81cca3ff8e7da609ca868f39e5709
Author: Jean-Marc Lasgouttes <lasgouttes at lyx.org>
Date:   Fri Mar 26 14:19:07 2021 +0100

    Fixup 4671214f: add comment
---
 src/BufferView.cpp |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/src/BufferView.cpp b/src/BufferView.cpp
index f9bbe23..308cf7f 100644
--- a/src/BufferView.cpp
+++ b/src/BufferView.cpp
@@ -3159,6 +3159,8 @@ void BufferView::caretPosAndDim(Point & p, Dimension & dim) const
 	} else {
 		Font const font = cur.real_current_font;
 		frontend::FontMetrics const & fm = theFontMetrics(font);
+		// lineWidth() can be 0 to mean 'thin line' on HiDpi, but the
+		// caret drawing code is not prepared for that.
 		dim.wid = max(fm.lineWidth(), 1);
 		dim.asc = fm.maxAscent();
 		dim.des = fm.maxDescent();