adding commas each in two sentences in order to make readers fully understand when reading index.md and to organize the context properly based on grammatical rules.
In order to move viewer related options from the global `PDFJS` object and into the initialization of the relevant components, we'll need to parse the hash parameters *before* calling `PDFViewerApplication._initializeViewerComponents`.
We're currently disabling range requests and streaming for a number of configurations. A couple of those will no longer be supported (with PDF.js version 2.0), hence we ought to be able to clean up the compatibility code slightly.
This compatibility code is only relevant for browsers that will no longer be supported (with PDF.js version 2.0), hence we ought to be able to remove it.
The only reason for adding this parameter in the first place, all the way back in PR 4074, was that the "maintain document position on zooming" feature was landed and backed out a couple of times before it finally stuck.
Hence it seemed, at the time, like a good idea to have a simple way to disable that behaviour. However, that was almost four years ago, and it's just not likely that we'd want/need to ever disable it now.
Furthermore I really cannot imagine why anyone would actually *want* to reset the position whenever zooming occurs, since it results in a quite annoying UX.
*So, to summarize:* Based on the above, I think that we should try to remove this parameter now. On the off chance that anyone complains, re-adding it shouldn't be difficult.
If the sidebar is resized such that the thumbnails are displayed in multiple columns, then scrolling the currently active thumbnail into view doesn't work correctly in some cases.
The reason is that the code in `PDFThumbnailViewer.scrollThumbnailIntoView` implicitly assumes that the thumbnails will be present in just *one* column. Since that may no longer be the case, it's not sufficient to simply check if the thumbnail is visible. Instead we must explicitly check that *all*, i.e. 100 percent, of the thumbnail is already visible, and otherwise scroll it into view.
*This patch is the result of me starting to look into moving parameters from `PDFJS` into `getDocument` and other API methods.*
When familiarizing myself with the code, the signatures of the various network streams seemed to be unnecessarily cumbersome since `disableRange` is currently handled separately from other parameters.
I'm assuming that the explanation for this is probably "for historical reasons", as is often the case. Hence I'd like to clean this up *before* we start the larger, and more invasive, `PDFJS` parameter re-factoring.
The interface of all of the "image" streams look kind of weird, and I'm actually a bit surprised that there hasn't been any errors because of it.
For example: None of them actually implement `readBlock` methods, and it seems more luck that anything else that we're not calling `getBytes()` (without providing a length) for those streams, since that would trigger a code-path in `getBytes` that assumes `readBlock` to exist.
To address this long-standing issue, the `ensureBuffer` methods are thus renamed to `readBlock`. Furthermore, the new `ensureBuffer` methods are now no-ops.
Finally, this patch also replaces `var` with `let` in a number of places.
By making use of modern CSS features, in this case [CSS variables](https://developer.mozilla.org/en-US/docs/Web/CSS/Using_CSS_variables), implementing sidebar resizing is actually quite simple. Not only will the amount of added code be fairly small, but it should also be easy to maintain since there's no need for complicated JavaScript hacks in order to update the CSS. Another benefit is that the JavaScript code doesn't need to make detailed assumptions about the exact structure of the HTML/CSS code.
Obviously this will not work in older browsers, such as IE, that lack support for CSS variables. In those cases sidebar resizing is simply disabled (via feature detection), and the resizing DOM element hidden, and the behaviour is thus *identical* to the current (fixed-width) sidebar.
However, considering the simplicity of the implementation, I really don't see why limiting this feature to "modern" browsers is a problem.
Finally, note that a few edge-cases meant that the patch is a bit larger than what the basic functionality would dictate. Among those is first of all proper RTL support, and secondly (automatic) resizing of the sidebar when the width of the *entire* viewer changes. Another, pre-existing, issue fixed here is the incomplete interface of `NullL10n`.
*Please note:* This patch has been successfully tested in both LTR and RTL viewer locales, in recent versions of Firefox and Chrome.
Fixes 2072.
https://eslint.org/docs/rules/no-var
Please note that two files were excluded:
1. `web/debugger.js`, since there's code in other files that currently depend on the global availability of code in `web/debugger.js`. Furthermore, since that file isn't used in production, doing a ES6 conversion probably isn't a priority.
2. `web/grab_to_pan.js`, since that file could be considered to be "external" code. We have made smaller changes to that file over the years, however doing a full ES6 `class` conversion might be a step too far!?
