[win32] Cache font height to prevent inconsistency

This commit adapts a limitation of handling fonts in the win32
implementation. When a font is initialized with FontData the height in
points of it and the desired target zoom will be used to calculate the
font height in pixels. When the FontData of an existing font is retrieved
the Logfont data from the OS together with the font zoom are used to
recalculate the original font height. As the conversion to pixels
involved rounding, the reversed calculation will result in a different
value than was used on creation time. With monitor-specific scaling
a font for a different zoom will be calculated on the basis of the
FontData of an existing base font. This can lead to font heights in
pixels that differ from the "correct" value. To solve this limitation
each font handle and its original font height in points are cached in a
static Map to recreate the original value when Font::getFontData is called.

Author: akoch-yatta

bundles/org.eclipse.swt/Eclipse SWT/win32/org/eclipse/swt/graphics/Font.java

@@ -14,6 +14,8 @@
 package org.eclipse.swt.graphics;
 
 
+import java.util.*;
+
 import org.eclipse.swt.*;
 import org.eclipse.swt.internal.*;
 import org.eclipse.swt.internal.win32.*;
@@ -55,6 +57,9 @@ public final class Font extends Resource {
 	 * (Warning: This field is platform dependent)
 	 */
 	int zoom;
+
+	private static Map<Long, Float> fontHeightCache = new HashMap<>();
+
 /**
  * Prevents uninitialized instances from being created outside the package.
  */
@@ -176,6 +181,7 @@ public Font(Device device, String name, int height, int style) {
 @Override
 void destroy() {
 	OS.DeleteObject(handle);
+	fontHeightCache.remove(handle);
 	handle = 0;
 }
 
@@ -213,7 +219,13 @@ public FontData[] getFontData() {
 	if (isDisposed()) SWT.error(SWT.ERROR_GRAPHIC_DISPOSED);
 	LOGFONT logFont = new LOGFONT ();
 	OS.GetObject(handle, LOGFONT.sizeof, logFont);
-	float heightInPoints = device.computePoints(logFont, handle, zoom);
+
+	float heightInPoints;
+	if (fontHeightCache.containsKey(handle)) {
+		heightInPoints = fontHeightCache.get(handle);
+	} else {
+		heightInPoints = device.computePoints(logFont, handle, zoom);
+	}
 	return new FontData[] {FontData.win32_new(logFont, heightInPoints)};
 }
 
@@ -240,6 +252,7 @@ void init (FontData fd) {
 	handle = OS.CreateFontIndirect(logFont);
 	logFont.lfHeight = lfHeight;
 	if (handle == 0) SWT.error(SWT.ERROR_NO_HANDLES);
+	fontHeightCache.put(handle, fd.height);
 }
 
 /**

bundles/org.eclipse.swt/Eclipse SWT/win32/org/eclipse/swt/internal/ScalingSWTFontRegistry.java

@@ -53,8 +53,6 @@ private Font getScaledFont(int targetZoom) {
 
 		private Font scaleFont(int zoom) {
 			FontData fontData = baseFont.getFontData()[0];
-			int baseZoom = computeZoom(fontData);
-			fontData.data.lfHeight = Math.round(1.0f * fontData.data.lfHeight * zoom / baseZoom);
 			Font scaledFont = Font.win32_new(device, fontData, zoom);
 			addScaledFont(zoom, scaledFont);
 			return scaledFont;
@@ -134,9 +132,8 @@ public Font getFont(FontData fontData, int zoom) {
 		if (fontKeyMap.containsKey(fontData)) {
 			container = fontKeyMap.get(fontData);
 		} else {
-			int calculatedZoom = computeZoom(fontData);
-			Font newFont = Font.win32_new(device, fontData, calculatedZoom);
-			container = new ScaledFontContainer(newFont, calculatedZoom);
+			Font newFont = Font.win32_new(device, fontData, zoom);
+			container = new ScaledFontContainer(newFont, zoom);
 			fontHandleMap.put(newFont.handle, container);
 			fontKeyMap.put(fontData, container);
 		}
@@ -167,15 +164,4 @@ private Font getOrCreateFont(ScaledFontContainer container, int zoom) {
 		}
 		return scaledFont;
 	}
-
-	private int computeZoom(FontData fontData) {
-		int pixelHeight = fontData.data.lfHeight;
-		float currentPointHeight = fontData.height;
-		if (pixelHeight == 0 || Math.abs(currentPointHeight) < 0.001) {
-			// if there is no font yet available, we use a defined zoom
-			return 100;
-		}
-		float pointHeightOn100 = -(pixelHeight / 96f * 72f);
-		return Math.round(100.0f * pointHeightOn100 / currentPointHeight);
-	}
 }