Re: Find On Page



I guess I need to go deeper into the layered design I mentioned in my last reply (the best person to explain the browser side of things is Marco Zehe from Mozilla Foundation):

To let NVDA navigate a website as though you are reading a document, NVDA employs what’s called a “tree interceptor”. A tree interceptor is a collection of elements that act like one large text area. Because a tree interceptor is a collection of elements and texts coming from these, it can include hundreds of different controls of varying roles (straight text, form fields, links, frames, web apps, video players, you name it). Using a tree to describe a complex control such as a web document is quite an interesting approach – after all, graphical user interface elements are organized like a tree or branches, with web documents consisting of a collection of smaller elements (and this is how HTML and ARIA coding is actually rendered on screen).

But tree interceptors are not enough. NVDA relies on three more materials to create a completely functioning browse mode implementation: virtual buffers, accessibility API’s, and a cursor manager (there are other elements involved, but these three are essential). Virtual buffers create building blocks for documents. Accessibility API’s and standards such as IAccessible and ARIA (Accessible Rich Internet Applications) enhance the “look and feel” of a browse mode document. Finally, cursor managers provide commands to move around the just created browse mode document.

The steps NVDA takes when creating a browse mode document are as follows:

  1. You start a web browser. NVDA can then load appropriate support modules based on the browser you are using.
  2. You open a new website.
  3. When NVDA detects that a new page is open, NVDA will either use API’s provided by the web browser to gather information about the just opened document (typically UIA browse mode implementations in Chromium Edge) or ask a DLL that ships with NVDA to gather info on the fly (Firefox is a good example of this).
  4. Whatever method is chosen, NVDA will read the document from top to bottom, constructing NVDA objects to represent elements found in the web document. At the same time, texts coming from these web elements are gathered into a single browse mode document (tree interceptor in some cases) to facilitate navigation, first letter commands, NVDA find, elements list and a whole host of features.


As for NVDA find command, NVDA will search text of the browse mode document (stored internally) and will place the cursor at the next occurrence of the searched term.

P.S. Marco, if you are here, may I ask if you can shed some light on browse mode/tree interceptor internals? I can speak of what NVDA does at the high level.





From: <> On Behalf Of Brian Vogel
Sent: Thursday, April 8, 2021 8:27 PM
Subject: Re: [nvda] Find On Page



          Before I start even trying to explain this, it would be helpful to know what browser or browsers you and Glenn are using and where either one of these finds is not working.

           I am going to try to make the distinction between a browser find and a screen reader find (and it doesn't matter whether it's JAWS or NVDA) as simple as I can.  I can assure you that I will be omitting scads of "under the hood" detail that someone far more knowledgeable about both browser internals and NVDA internals can delve in to if they so choose.

            A browser find focuses on what can be seen on a page that's a part of the page text.  That is generally limited to actual text as well as text used for click-through links and labels.  But text on controls, like buttons, checkboxes, etc., will very often not be found using a browser find.  Much of this depends on how sloppy the page coders have been about how certain controls are written and what's exposed to a browser find versus a screen reader find.  I also believe that a browser find does not examine the virtual cursor used by the screen reader while a screen reader find does just that.

             As a result, there can be differences in not only what can be found by either one, but exactly where the screen reader focus is after each is done.

             I tend to favor the screen reader find when someone's using a screen reader simply because it tends to find certain things that a browser find doesn't, and you more often have focus on the thing just found, consistently, with a screen reader find.

             I'm actually hoping someone with way more "under the hood" knowledge will chime in and probably bore some of us silly getting into the actual differences between how a browser find and screen reader find works and can explain the discrepancies not only in what each can find but in where focus lies after each.  I have never been able to come up with any precise way of describing what's different between the two.

Brian - Windows 10 Pro, 64-Bit, Version 20H2, Build 19042  

Always remember others may hate you but those who hate you don't win unless you hate them.  And then you destroy yourself.

       ~ Richard M. Nixon


Join to automatically receive all group messages.