Move text layer scaling logic to CSS

This commit moves all the logic to scale up&down `<span>`s in the text
layer, introduced in #18283, to CSS.

The motivation for this change is that #18283 is still not enough for
all cases. That PR fixed the problem in Chrome&Firefox desktop, which
allow users to set an actual minimum font size in the browser settings.
However, other browsers (e.g. the Chrome-based WebView on Android) have
more complex logic and they scale up small text rather than simply
applying a minimum.

A workaround for that behavior is probably out of scope for PDF.js
itself as it only affects not officially supported platforms. However,
having access to the actual expected font height (through
`--font-height`) allows embedders of PDF.js to implement a workaround by
themselves.

Author: nicolo-ribaudo

src/display/text_layer.js

@@ -128,6 +128,7 @@ class TextLayer {
     this.#pageHeight = pageHeight;
 
     TextLayer.#ensureMinFontSizeComputed();
+    container.style.setProperty("--min-font-size", TextLayer.#minFontSize);
 
     setLayerDimensions(container, viewport);
 
@@ -342,7 +343,6 @@ class TextLayer {
       top = tx[5] - fontAscent * Math.cos(angle);
     }
 
-    const scaleFactorStr = "calc(var(--total-scale-factor) *";
     const divStyle = textDiv.style;
     // Setting the style properties individually, rather than all at once,
     // should be OK since the `textDiv` isn't appended to the document yet.
@@ -351,14 +351,10 @@ class TextLayer {
       divStyle.top = `${((100 * top) / this.#pageHeight).toFixed(2)}%`;
     } else {
       // We're in a marked content span, hence we can't use percents.
-      divStyle.left = `${scaleFactorStr}${left.toFixed(2)}px)`;
-      divStyle.top = `${scaleFactorStr}${top.toFixed(2)}px)`;
+      divStyle.left = `calc(var(--total-scale-factor) * ${left.toFixed(2)}px)`;
+      divStyle.top = `calc(var(--total-scale-factor) * ${top.toFixed(2)}px)`;
     }
-    // We multiply the font size by #minFontSize, and then #layout will
-    // scale the element by 1/#minFontSize. This allows us to effectively
-    // ignore the minimum font size enforced by the browser, so that the text
-    // layer <span>s can always match the size of the text in the canvas.
-    divStyle.fontSize = `${scaleFactorStr}${(TextLayer.#minFontSize * fontHeight).toFixed(2)}px)`;
+    divStyle.setProperty("--font-height", `${fontHeight.toFixed(2)}px`);
     divStyle.fontFamily = fontFamily;
 
     textDivProperties.fontSize = fontHeight;
@@ -421,11 +417,6 @@ class TextLayer {
     const { div, properties, ctx } = params;
     const { style } = div;
 
-    let transform = "";
-    if (TextLayer.#minFontSize > 1) {
-      transform = `scale(${1 / TextLayer.#minFontSize})`;
-    }
-
     if (properties.canvasWidth !== 0 && properties.hasText) {
       const { fontFamily } = style;
       const { canvasWidth, fontSize } = properties;
@@ -435,14 +426,11 @@ class TextLayer {
       const { width } = ctx.measureText(div.textContent);
 
       if (width > 0) {
-        transform = `scaleX(${(canvasWidth * this.#scale) / width}) ${transform}`;
+        style.setProperty("--scale-x", (canvasWidth * this.#scale) / width);
       }
     }
     if (properties.angle !== 0) {
-      transform = `rotate(${properties.angle}deg) ${transform}`;
-    }
-    if (transform.length > 0) {
-      style.transform = transform;
+      style.setProperty("--rotate", `${properties.angle}deg`);
     }
   }
 

test/unit/text_layer_spec.js

@@ -101,11 +101,12 @@ describe("textLayer", function () {
     const getTransform = container => {
       const transform = [];
 
-      for (const span of container.childNodes) {
-        const t = span.style.transform;
-        expect(t).toMatch(/^scaleX\([\d.]+\)$/);
-
-        transform.push(t);
+      for (const { style } of container.childNodes) {
+        transform.push({
+          fontHeight: style.getPropertyValue("--font-height"),
+          scaleX: style.getPropertyValue("--scale-x"),
+          rotate: style.getPropertyValue("--rotate"),
+        });
       }
       return transform;
     };

web/text_layer_builder.css

@@ -38,9 +38,25 @@
     transform-origin: 0% 0%;
   }
 
+  /* We multiply the font size by --min-font-size, and then scale the text
+   * elements by 1/--min-font-size. This allows us to effectively ignore the
+   * minimum font size enforced by the browser, so that the text layer <span>s
+   * can always match the size of the text in the canvas. */
+  --min-font-size: 1;
+  --text-scale-factor: calc(var(--total-scale-factor) * var(--min-font-size));
+  --min-font-size-inv: calc(1 / var(--min-font-size));
+
   > :not(.markedContent),
   .markedContent span:not(.markedContent) {
     z-index: 1;
+
+    --font-height: 0; /* set by text_layer.js */
+    font-size: calc(var(--text-scale-factor) * var(--font-height));
+
+    --scale-x: 1;
+    --rotate: 0deg;
+    transform: rotate(var(--rotate)) scaleX(var(--scale-x))
+      scale(var(--min-font-size-inv));
   }
 
   /* Only necessary in Google Chrome, see issue 14205, and most unfortunately