Make negative settings for extra ascent and descent safer by ensuring calculated

ascent and thus line height is at least 1 pixel.
Allow 1 point font minimum instead of 2 as this may help with document maps.
Document negative extra ascent and descent.
Use XYPOSITION for ascent and descent for more coherent types and fewer casts
but floor the values when retrieving from Surface so behaviour is unchanged.

4 files changed, 18 insertions, 12 deletions

diff --git a/src/EditView.cxx b/src/EditView.cxx
index 0f69dd7b3..732a0f72e 100644
--- a/src/EditView.cxx
+++ b/src/EditView.cxx
@@ -2700,7 +2700,7 @@ Sci::Position EditView::FormatRange(bool draw, const RangeToFormat *pfr, Surface
 				vsPrint.styles[StyleLineNumber].font.get(), number);
 			surface->FlushCachedState();
 			surface->DrawTextNoClip(rcNumber, vsPrint.styles[StyleLineNumber].font.get(),
-				static_cast<XYPOSITION>(ypos + vsPrint.maxAscent), number,
+				ypos + vsPrint.maxAscent, number,
 				vsPrint.styles[StyleLineNumber].fore,
 				vsPrint.styles[StyleLineNumber].back);
 		}
diff --git a/src/Style.h b/src/Style.h
index a3ce336ed..14112e092 100644
--- a/src/Style.h
+++ b/src/Style.h
@@ -27,8 +27,8 @@ struct FontSpecification {
 };
 
 struct FontMeasurements {
-	unsigned int ascent = 1;
-	unsigned int descent = 1;
+	XYPOSITION ascent = 1;
+	XYPOSITION descent = 1;
 	XYPOSITION capitalHeight = 1;	// Top of capital letter to baseline: ascent - internal leading
 	XYPOSITION aveCharWidth = 1;
 	XYPOSITION spaceWidth = 1;
diff --git a/src/ViewStyle.cxx b/src/ViewStyle.cxx
index 8c3872611..ad1340d0d 100644
--- a/src/ViewStyle.cxx
+++ b/src/ViewStyle.cxx
@@ -8,6 +8,7 @@
 #include <cstddef>
 #include <cassert>
 #include <cstring>
+#include <cmath>
 
 #include <stdexcept>
 #include <string>
@@ -48,16 +49,20 @@ bool MarginStyle::ShowsFolding() const noexcept {
 void FontRealised::Realise(Surface &surface, int zoomLevel, Technology technology, const FontSpecification &fs, const char *localeName) {
 	PLATFORM_ASSERT(fs.fontName);
 	measurements.sizeZoomed = fs.size + zoomLevel * FontSizeMultiplier;
-	if (measurements.sizeZoomed <= 2 * FontSizeMultiplier)	// Hangs if sizeZoomed <= 1
-		measurements.sizeZoomed = 2 * FontSizeMultiplier;
+	if (measurements.sizeZoomed <= FontSizeMultiplier)	// May fail if sizeZoomed < 1
+		measurements.sizeZoomed = FontSizeMultiplier;
 
 	const float deviceHeight = static_cast<float>(surface.DeviceHeightFont(measurements.sizeZoomed));
 	const FontParameters fp(fs.fontName, deviceHeight / FontSizeMultiplier, fs.weight,
 		fs.italic, fs.extraFontFlag, technology, fs.characterSet, localeName);
 	font = Font::Allocate(fp);
 
-	measurements.ascent = static_cast<unsigned int>(surface.Ascent(font.get()));
-	measurements.descent = static_cast<unsigned int>(surface.Descent(font.get()));
+	// floor here is historical as platform layers have tweaked their values to match.
+	// ceil would likely be better to ensure (nearly) all of the ink of a character is seen
+	// but that would require platform layer changes.
+	measurements.ascent = std::floor(surface.Ascent(font.get()));
+	measurements.descent = std::floor(surface.Descent(font.get()));
+
 	measurements.capitalHeight = surface.Ascent(font.get()) - surface.InternalLeading(font.get());
 	measurements.aveCharWidth = surface.AverageCharWidth(font.get());
 	measurements.spaceWidth = surface.WidthText(font.get(), " ");
@@ -356,9 +361,10 @@ void ViewStyle::Refresh(Surface &surface, int tabInChars) {
 	maxAscent = 1;
 	maxDescent = 1;
 	FindMaxAscentDescent();
-	maxAscent += extraAscent;
-	maxDescent += extraDescent;
-	lineHeight = maxAscent + maxDescent;
+	// Ensure reasonable values: lines less than 1 pixel high will not work 
+	maxAscent = std::max(1.0, maxAscent + extraAscent);
+	maxDescent = std::max(0.0, maxDescent + extraDescent);
+	lineHeight = lround(maxAscent + maxDescent);
 	lineOverlap = lineHeight / 10;
 	if (lineOverlap < 2)
 		lineOverlap = 2;
diff --git a/src/ViewStyle.h b/src/ViewStyle.h
index edebee7ce..f6d1bdbb4 100644
--- a/src/ViewStyle.h
+++ b/src/ViewStyle.h
@@ -122,8 +122,8 @@ public:
 	Scintilla::Technology technology;
 	int lineHeight;
 	int lineOverlap;
-	unsigned int maxAscent;
-	unsigned int maxDescent;
+	XYPOSITION maxAscent;
+	XYPOSITION maxDescent;
 	XYPOSITION aveCharWidth;
 	XYPOSITION spaceWidth;
 	XYPOSITION tabWidth;