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`](5b4c2fe1a8/src/display/text_layer.js (L396-L428)) method.
*Order of events, for parallel textLayer rendering:*
1. Call [`render`](5b4c2fe1a8/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](5b4c2fe1a8/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.
This is very old code, and predates e.g. the introduction of JavaScript classes, which creates unnecessarily unwieldy code in the viewer.
By introducing a new `TextLayer` class in the API, similar to how e.g. the `AnnotationLayer` looks, we're able to keep most parameters on the class-instance itself. This removes the need to manually track them in the viewer, and simplifies the call-sites.
This also removes the `numTextDivs` parameter from the "textlayerrendered" event, since that's only added to support default-viewer functionality that no longer exists.
Finally we try, as far as possible, to polyfill the old `renderTextLayer` and `updateTextLayer` functions since they are exposed in the library API.
For *simple* invocations of `renderTextLayer` the behaviour should thus be the same, with only a warning printed in the console.
The only reason that this code still accepts `TextContent` is for backward-compatibility purposes, so we can simplify the implementation by always using a `ReadableStream` internally.
With the changes in the previous patch the `isNodeJS`-helper no longer needs to live in its own file, which helps get rid of a closure in the *built* files.
Unfortunately I don't believe that we can simply add a default `--scale-factor` CSS-variable to the `container`-element, since that might not be entirely appropriate/correct in all cases.[1]
However, we can at least print a console-error to hopefully make this situation more apparent to users. (This is purposely not using the `warn` helper-function, since those messages can be disabled.)
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[1] One example is in our reference-tests, where we don't need to add it to the `container`-element itself.
Rather than handling these parameters separately, which is a left-over from back when streaming of textContent was originally added, we can simply pass either data directly to the `TextLayer` and let it handle things accordingly.
Also, improves a few JSDoc comments and `typedef`-imports.
The idea is just to resuse what we got on the first draw.
Now, we only update the scaleX of the different spans and the other values
are dependant of --scale-factor.
Move some properties in the CSS in order to avoid any updates in JS.
While `TextLayerRenderTask` apparently makes sense in TypeScript environments, given that it's being returned by the `renderTextLayer`-function in the API, we really don't want to extend the *public* API by simply exporting the class directly in `src/pdf.js` since it should never be called/initialized manually.
Hence we follow the same pattern as in PR 14013, and add some very basic unit-tests to ensure that `renderTextLayer` always returns a `TextLayerRenderTask`-instance as expected.
