Improve layout when a text run contains left-to-right and right-to-left ranges.

2 files changed, 28 insertions, 0 deletions

diff --git a/doc/ScintillaHistory.html b/doc/ScintillaHistory.html
index 7c8e77510..ea9cb4532 100644
--- a/doc/ScintillaHistory.html
+++ b/doc/ScintillaHistory.html
@@ -598,6 +598,12 @@
 	Add APIs for saving and restoring undo history.
 	</li>
 	<li>
+	For GTK, when laying out text, detect runs with both left-to-right and right-to-left ranges and divide into an ASCII prefix and
+	more complex suffix. Lay out the ASCII prefix in the standard manner but, for the suffix, measure the whole width
+	and spread that over the suffix bytes. This produces more usable results where the caret moves over the ASCII prefix correctly
+	and over the suffix reasonably but not accurately.
+	</li>
+	<li>
 	For ScintillaEdit on Qt, fix reference from ScintillaDocument to Document to match change in 5.4.1
 	using IDocumentEditable for SCI_GETDOCPOINTER and SCI_SETDOCPOINTER.
 	</li>
diff --git a/gtk/PlatGTK.cxx b/gtk/PlatGTK.cxx
index 283dd7a4b..e7d6abd3d 100755
--- a/gtk/PlatGTK.cxx
+++ b/gtk/PlatGTK.cxx
@@ -36,6 +36,7 @@
 
 #include "Scintilla.h"
 #include "ScintillaWidget.h"
+#include "CharacterType.h"
 #include "XPM.h"
 #include "UniConversion.h"
 
@@ -1085,6 +1086,27 @@ void SurfaceImpl::MeasureWidthsUTF8(const Font *font_, std::string_view text, XY
 		}
 		while (!iti.finished) {
 			iti.Next();
+			if (iti.curIndex < i) {
+				// Backwards movement indicater bidirectional.
+				// Divide into ASCII prefix and non-ASCII suffix as this is common case
+				// and produces accurate positions for the ASCII prefix.
+				size_t lenASCII=0;
+				while (lenASCII<text.length() && IsASCII(text[lenASCII])) {
+					lenASCII++;
+				}
+				const std::string_view asciiPrefix = text.substr(0, lenASCII);
+				const std::string_view bidiSuffix = text.substr(lenASCII);
+				// Recurse for ASCII prefix.
+				MeasureWidthsUTF8(font_, asciiPrefix, positions);
+				// Measure the whole bidiSuffix and spread its width evenly
+				const XYPOSITION endASCII = positions[lenASCII-1];
+				const XYPOSITION widthBidi = WidthText(font_, bidiSuffix);
+				const XYPOSITION widthByteBidi = widthBidi / bidiSuffix.length();
+				for (size_t bidiPos=0; bidiPos<bidiSuffix.length(); bidiPos++) {
+					positions[bidiPos+lenASCII] = endASCII + widthByteBidi * (bidiPos + 1);
+				}
+				return;
+			}
 			const int places = iti.curIndex - i;
 			while (i < iti.curIndex) {
 				// Evenly distribute space among bytes of this cluster.