Client: Don't crash with long window titles using UTF-16 characters

Previously, we set the max length in QString character length, which means
UTF-16 characters (of potentially three bytes) counts as one character. The max
limit of libwayland, however, is in bytes (and the string itself is converted
to UTF-8).

Fix it by dividing the character limit by three because in the worst case each
UTF-16 character will use three bytes when UTF-8 encoded.

Fixes: QTBUG-78478
Change-Id: Idf4721894e0fe6f3cd92bdc6ada7b0ea4199ea63
(cherry picked from commit 1ed0782e93dafb0a3d5ef3a02c9c3999825817ee)
Reviewed-by: Paul Olav Tvete <paul.tvete@qt.io>

1 files changed, 4 insertions, 2 deletions

diff --git a/src/client/qwaylandwindow.cpp b/src/client/qwaylandwindow.cpp
index fba6cc96c..3cd7892a7 100644
--- a/src/client/qwaylandwindow.cpp
+++ b/src/client/qwaylandwindow.cpp
@@ -309,8 +309,10 @@ void QWaylandWindow::setWindowTitle(const QString &title)
         const QString formatted = formatWindowTitle(title, separator);
 
         const int libwaylandMaxBufferSize = 4096;
-        // Some parts of the buffer is used for metadata, so subtract 100 to be on the safe side
-        const int maxLength = libwaylandMaxBufferSize - 100;
+        // Some parts of the buffer is used for metadata, so subtract 100 to be on the safe side.
+        // Also, QString is in utf-16, which means that in the worst case each character will be
+        // three bytes when converted to utf-8 (which is what libwayland uses), so divide by three.
+        const int maxLength = libwaylandMaxBufferSize / 3 - 100;
 
         auto truncated = QStringRef(&formatted).left(maxLength);
         if (truncated.length() < formatted.length()) {