Commits on Sep 11, 2024

1. Ensure that textLayers can be rendered in parallel, without interferi… …

   …ng with each other
   
   Note that the textContent is returned in "chunks" from the API, through the use of `ReadableStream`s, and on the main-thread we're (normally) using just one temporary canvas in order to measure the size of the textLayer `span`s; see the [`#layout`](https://github.com/mozilla/pdf.js/blob/5b4c2fe1a845169ac2b4f8f6335337c434077637/src/display/text_layer.js#L396-L428) method.
   
   *Order of events, for parallel textLayer rendering:*
    1. Call [`render`](https://github.com/mozilla/pdf.js/blob/5b4c2fe1a845169ac2b4f8f6335337c434077637/src/display/text_layer.js#L155-L177) of the textLayer for page A.
    2. Immediately call `render` of the textLayer for page B.
    3. The first text-chunk for pageA arrives, and it's parsed/layout which means updating the cached [fontSize/fontFamily](https://github.com/mozilla/pdf.js/blob/5b4c2fe1a845169ac2b4f8f6335337c434077637/src/display/text_layer.js#L409-L413) for the textLayer of page A.
    4. The first text-chunk for pageB arrives, which means updating the cached fontSize/fontFamily *for the textLayer of page B* since this data is unique to each `TextLayer`-instance.
    5. The second text-chunk for pageA arrives, and we don't update the canvas-font since the cached fontSize/fontFamily still apply from step 3 above.
   
   Where this potentially breaks down is between the last steps, since we're using just one temporary canvas for all measurements but have *individual* fontSize/fontFamily caches for each textLayer.
   Hence it's possible that the canvas-font has actually changed, despite the cached values suggesting otherwise, and to address this we instead cache the fontSize/fontFamily globally through a new (static) helper method.
   
   *Note:* Includes a basic unit-test, using dummy text-content, which fails on `master` and passes with this patch.
   
   Finally, pun intended, ensure that temporary textLayer-data is cleared *before* the `render`-promise resolves to avoid any intermittent problems in the unit-tests.

   

   Snuffleupagus committed Sep 11, 2024
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