Fix px unit page size measurement (#3921)

- Make correct px scaling (72/96) the default behavior
- Fixes incorrect page size measurements when using px unit with format strings
- A4 format now correctly measures as 793.71 x 1122.52 px instead of 446.46 x 631.42 px
- Add px_scaling_legacy hotfix for backward compatibility with old (incorrect) behavior
- Update documentation to reflect the change

Author: AyushCoder9

HOTFIX_README.md

@@ -16,25 +16,39 @@ new jsPDF({
 
 # Active Hotfixes
 
-## px_scaling
+## px_scaling_legacy
 
 ### Applies To
 
 jsPDF Core
 
 ### Description
 
-When supplying 'px' as the unit for the PDF, the internal scaling factor was being miscalculated making drawn components
-larger than they should be. Enabling this hotfix will correct this scaling calculation and items will be drawn to the
-correct scale.
+For backward compatibility, this hotfix restores the old (incorrect) pixel scaling behavior where scaleFactor = 96/72.
+By default, jsPDF now uses the correct pixel scaling (scaleFactor = 72/96) which matches the CSS standard where
+1px = 1/96in and 1pt = 1/72in, resulting in 1px = 72/96 pt.
 
 ### To Enable
 
-To enable this hotfix, supply a 'hotfixes' array to the options object in the jsPDF constructor function, and add the
-string 'px_scaling' to this array.
+To enable this hotfix (restore old behavior), supply a 'hotfixes' array to the options object in the jsPDF constructor function, and add the
+string 'px_scaling_legacy' to this array.
 
 # Accepted Hotfixes
 
+## px_scaling
+
+### Applies To
+
+jsPDF Core
+
+### Description
+
+When supplying 'px' as the unit for the PDF, the internal scaling factor was being miscalculated making drawn components
+larger than they should be. This hotfix corrected the scaling calculation so items are drawn to the correct scale.
+
+This is now the default behavior as of the fix for issue #3921. The correct scaling (72/96) is used by default.
+For backward compatibility with the old incorrect scaling (96/72), use the 'px_scaling_legacy' hotfix.
+
 ## scale_text
 
 ### Applies To

src/jspdf.js

@@ -179,7 +179,8 @@ function TilingPattern(boundingBox, xStep, yStep, gState, matrix) {
  * @param {string} [options.orientation=portrait] - Orientation of the first page. Possible values are "portrait" or "landscape" (or shortcuts "p" or "l").<br />
  * @param {string} [options.unit=mm] Measurement unit (base unit) to be used when coordinates are specified.<br />
  * Possible values are "pt" (points), "mm", "cm", "in", "px", "pc", "em" or "ex". Note that in order to get the correct scaling for "px"
- * units, you need to enable the hotfix "px_scaling" by setting options.hotfixes = ["px_scaling"].
+ * units, the correct scaling (72/96) is now the default. For backward compatibility with the old incorrect scaling (96/72),
+ * you can enable the hotfix "px_scaling_legacy" by setting options.hotfixes = ["px_scaling_legacy"].
  * @param {string/Array} [options.format=a4] The format of the first page. Can be:<ul><li>a0 - a10</li><li>b0 - b10</li><li>c0 - c10</li><li>dl</li><li>letter</li><li>government-letter</li><li>legal</li><li>junior-legal</li><li>ledger</li><li>tabloid</li><li>credit-card</li></ul><br />
  * Default is "a4". If you want to use your own format just pass instead of one of the above predefined formats the size as an number-array, e.g. [595.28, 841.89]
  * @param {boolean} [options.putOnlyUsedFonts=false] Only put fonts into the PDF, which were used.
@@ -2713,12 +2714,15 @@ function jsPDF(options) {
     if (typeof parmFormat === "string") {
       dimensions = getPageFormat(parmFormat.toLowerCase());
       if (Array.isArray(dimensions)) {
+        // Dimensions from getPageFormat are in points
+        // beginPage expects dimensions in points, so we store them as-is
         width = dimensions[0];
         height = dimensions[1];
       }
     }
 
     if (Array.isArray(parmFormat)) {
+      // Array format values are in user units, convert to points for storage
       width = parmFormat[0] * scaleFactor;
       height = parmFormat[1] * scaleFactor;
     }
@@ -3221,10 +3225,13 @@ function jsPDF(options) {
       scaleFactor = 72;
       break;
     case "px":
-      if (hasHotfix("px_scaling") == true) {
-        scaleFactor = 72 / 96;
-      } else {
+      // Default to correct px scaling (72/96 = 0.75)
+      // This matches CSS standard: 1px = 1/96in, 1pt = 1/72in, so 1px = 72/96 pt
+      // The old incorrect scaling (96/72) can be enabled via hotfix "px_scaling_legacy" for backward compatibility
+      if (hasHotfix("px_scaling_legacy") == true) {
         scaleFactor = 96 / 72;
+      } else {
+        scaleFactor = 72 / 96;
       }
       break;
     case "pc":