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.

1 files changed, 13 insertions, 7 deletions

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;