fix: correcting Assistant metrics (#7758)

* Changed Assistant metrics to the corrrect ones from OS/2 table

* Adding more information about font metrics

* Adding branded types to avoid future mistakes

packages/excalidraw/CHANGELOG.md

@@ -27,7 +27,7 @@ Please add the latest change on the top under the correct section.
 
 - `ExcalidrawEmbeddableElement.validated` was removed and moved to private editor state. This should largely not affect your apps unless you were reading from this attribute. We keep validating embeddable urls internally, and the public [`props.validateEmbeddable`](https://docs.excalidraw.com/docs/@excalidraw/excalidraw/api/props#validateembeddable) still applies. [#7539](https://github.com/excalidraw/excalidraw/pull/7539)
 
-- `ExcalidrawTextElement.baseline` was removed and replaced with a vertical offset computation based on font metrics, performed on each text element re-render.
+- `ExcalidrawTextElement.baseline` was removed and replaced with a vertical offset computation based on font metrics, performed on each text element re-render. In case of custom font usage, extend the `FONT_METRICS` object with the related properties.
 
 - Create an `ESM` build for `@excalidraw/excalidraw`. The API is in progress and subject to change before stable release. There are some changes on how the package will be consumed
 

packages/excalidraw/element/textElement.ts

@@ -31,7 +31,7 @@ import { getElementAbsoluteCoords } from ".";
 import { getSelectedElements } from "../scene";
 import { isHittingElementNotConsideringBoundingBox } from "./collision";
 
-import { ExtractSetType } from "../utility-types";
+import { ExtractSetType, MakeBrand } from "../utility-types";
 import {
   resetOriginalContainerCache,
   updateOriginalContainerCache,
@@ -928,38 +928,50 @@ const DEFAULT_LINE_HEIGHT = {
   [FONT_FAMILY.Cascadia]: 1.2 as ExcalidrawTextElement["lineHeight"],
 };
 
-type FontMetrics = {
-  unitsPerEm: number; // head.unitsPerEm
-  ascender: number; // sTypoAscender
-  descender: number; // sTypoDescender
-};
+/** OS/2 sTypoAscender, https://learn.microsoft.com/en-us/typography/opentype/spec/os2#stypoascender */
+type sTypoAscender = number & MakeBrand<"sTypoAscender">;
+
+/** OS/2 sTypoDescender, https://learn.microsoft.com/en-us/typography/opentype/spec/os2#stypodescender */
+type sTypoDescender = number & MakeBrand<"sTypoDescender">;
+
+/** head.unitsPerEm, usually either 1000 or 2048 */
+type unitsPerEm = number & MakeBrand<"unitsPerEm">;
 
 /**
  * Hardcoded metrics for default fonts, read by https://opentype.js.org/font-inspector.html.
- * For custom fonts, read these metrics on load and extend this object.
+ * For custom fonts, read these metrics from OS/2 table and extend this object.
+ *
+ * WARN: opentype does NOT open WOFF2 correctly, make sure to convert WOFF2 to TTF first.
  */
-const FONT_METRICS = {
+export const FONT_METRICS: Record<
+  number,
+  {
+    unitsPerEm: number;
+    ascender: sTypoAscender;
+    descender: sTypoDescender;
+  }
+> = {
   [FONT_FAMILY.Virgil]: {
-    unitsPerEm: 1000,
-    ascender: 886,
-    descender: -374,
+    unitsPerEm: 1000 as unitsPerEm,
+    ascender: 886 as sTypoAscender,
+    descender: -374 as sTypoDescender,
   },
   [FONT_FAMILY.Helvetica]: {
-    unitsPerEm: 2048,
-    ascender: 1577,
-    descender: -471,
+    unitsPerEm: 2048 as unitsPerEm,
+    ascender: 1577 as sTypoAscender,
+    descender: -471 as sTypoDescender,
   },
   [FONT_FAMILY.Cascadia]: {
-    unitsPerEm: 2048,
-    ascender: 1977,
-    descender: -480,
+    unitsPerEm: 2048 as unitsPerEm,
+    ascender: 1977 as sTypoAscender,
+    descender: -480 as sTypoDescender,
   },
   [FONT_FAMILY.Assistant]: {
-    unitsPerEm: 1000,
-    ascender: 1050,
-    descender: -500,
+    unitsPerEm: 1000 as unitsPerEm,
+    ascender: 1021 as sTypoAscender,
+    descender: -287 as sTypoDescender,
   },
-} as Record<number, FontMetrics>;
+};
 
 export const getDefaultLineHeight = (fontFamily: FontFamilyValues) => {
   if (fontFamily in DEFAULT_LINE_HEIGHT) {