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.
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