[Regression] Initial textLayer rendering not correct after PR 15722

[Snuffleupagus]

Attach (recommended) or Link to PDF file here:

Configuration:

• Web browser and its version: N/A
• Operating system and its version: N/A
• PDF.js version: Any version after PR [api-minor] Refactor the text layer code in order to avoid to recompute it on each draw #15722
• Is a browser extension: N/A

Steps to reproduce the problem:
After PR #15722, the textLayer will no longer have correct dimensions unless zooming has occurred at least once. This was found when testing the "pageviewer" component-example locally, see https://github.com/mozilla/pdf.js/tree/master/examples/components

To more easily reproduce this directly in the default viewer, apply the following diff and load http://localhost:8888/web/viewer.html#disableHistory=true&textLayer=visible:

diff --git a/web/app.js b/web/app.js
index e79ba7b5c..2d37c9319 100644
--- a/web/app.js
+++ b/web/app.js
@@ -1638,8 +1638,9 @@ const PDFViewerApplication = {
     if (!this.pdfViewer.currentScaleValue) {
       // Scale was not initialized: invalid bookmark or scale was not specified
.
       // Setting the default one.
-      this.pdfViewer.currentScaleValue = DEFAULT_SCALE_VALUE;
+      // this.pdfViewer.currentScaleValue = DEFAULT_SCALE_VALUE;
     }
+    this.pdfViewer.update();
   },

What is the expected behavior? (add screenshot)
Correct initial textLayer dimensions, as in:

What went wrong? (add screenshot)
Wrong initial textLayer dimensions, as in:

Link to a viewer (if hosted on a site other than mozilla.github.io/pdf.js or as Firefox/Chrome extension): N/A

/cc @calixteman

[Snuffleupagus]

One way to address this may be to always set the following by default in the PDFViewer/PDFPageView constructors, so if that makes sense overall I can submit a patch!

docStyle.setProperty("--scale-factor", PixelsPerInch.PDF_TO_CSS_UNITS);

[calixteman]

Yep it's what I found too (i.e. the fact that the scale-factor is wrong).
I'd say that this bug just highlights something which was already wrong.
Let's go for your fix :).