[Win32] Fix font metrics being scaled by GC transform for fallback fonts

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

@@ -680,7 +680,22 @@ static long createGdipFont(long hDC, long hFont, long graphics, long fontCollect
 		if (outFont != null && font != 0) {
 			long hHeap = OS.GetProcessHeap();
 			long pLogFont = OS.HeapAlloc(hHeap, OS.HEAP_ZERO_MEMORY, LOGFONT.sizeof);
-			Gdip.Font_GetLogFontW(font, graphics, pLogFont);
+			if (graphics != 0) {
+				// Font_GetLogFontW applies the graphics's world transform when converting the
+				// font size to LOGFONT units. A non-identity user transform would cause lfHeight
+				// to be scaled, producing an incorrectly-sized GDI font for GetTextMetrics.
+				// Temporarily set identity so the LOGFONT reflects device-pixel metrics.
+				long savedMatrix = Gdip.Matrix_new(1, 0, 0, 1, 0, 0);
+				Gdip.Graphics_GetTransform(graphics, savedMatrix);
+				long identityMatrix = Gdip.Matrix_new(1, 0, 0, 1, 0, 0);
+				Gdip.Graphics_SetTransform(graphics, identityMatrix);
+				Gdip.Font_GetLogFontW(font, graphics, pLogFont);
+				Gdip.Graphics_SetTransform(graphics, savedMatrix);
+				Gdip.Matrix_delete(identityMatrix);
+				Gdip.Matrix_delete(savedMatrix);
+			} else {
+				Gdip.Font_GetLogFontW(font, graphics, pLogFont);
+			}
 			outFont[0] = OS.CreateFontIndirect(pLogFont);
 			OS.HeapFree(hHeap, 0, pLogFont);
 		}

tests/org.eclipse.swt.tests/JUnit Tests/org/eclipse/swt/tests/junit/Test_org_eclipse_swt_graphics_GC.java

@@ -730,6 +730,45 @@ public void test_getFontMetrics() {
 	assertTrue(fm.getHeight() > 0);
 }
 
+/**
+ * Verifies that applying a transform to a GC does not affect the font metrics
+ * when the font does not exist on the current platform and a fallback font is
+ * used instead.
+ *
+ * @see <a href="https://github.com/eclipse-platform/eclipse.platform.swt/issues/2978">Issue 2978</a>
+ */
+@Test
+public void test_getFontMetrics_notAffectedByTransform() {
+	Font font = new Font(display, "NonExistentSWTTestFont", 24, SWT.NORMAL);
+	Transform scaleTransform = new Transform(display, 3.0f, 0.0f, 0.0f, 3.0f, 0.0f, 0.0f);
+	try {
+		gc.setFont(font);
+		gc.setTransform(scaleTransform);
+		double metricsWithTransform = gc.getFontMetrics().getAverageCharacterWidth();
+
+		gc.setTransform(null);
+		double metricsAfterClearingTransform = gc.getFontMetrics().getAverageCharacterWidth();
+
+		gc.setFont(font);
+		double metricsAfterReSettingFont = gc.getFontMetrics().getAverageCharacterWidth();
+
+		gc.setTransform(scaleTransform);
+		double metricsWithTransformAgain = gc.getFontMetrics().getAverageCharacterWidth();
+
+		assertAll(
+			() -> assertEquals(metricsWithTransform, metricsAfterClearingTransform, 0,
+					"Font metrics must not change after clearing the transform"),
+			() -> assertEquals(metricsAfterClearingTransform, metricsAfterReSettingFont, 0,
+					"Font metrics must not change when re-setting the font without transform"),
+			() -> assertEquals(metricsAfterReSettingFont, metricsWithTransformAgain, 0,
+					"Font metrics must not be scaled when the transform is re-applied")
+		);
+	} finally {
+		scaleTransform.dispose();
+		font.dispose();
+	}
+}
+
 @Test
 public void test_getStyle() {
 	Canvas canvas = new Canvas(shell, SWT.NULL);