1659273 - Static clone documents used for printing/preview should ignore prefs changes that would modify styling

Status: NEW

(Core :: Printing: Output, defect)

Description

[Jonathan Kew [:jfkthame]]

Modifying prefs such as default fonts can have undesirable results for print or print-preview documents that are currently alive. It looks like in response to a change such as updated default fonts, we restyle the document but do not reflow it. The result can be badly broken rendering.

I'm inclined to think static-clone documents should ignore such pref changes altogether. For a document that is in the process of being printed, it doesn't make sense to restyle and reflow in the middle of the print job; this would create a discontinuity, at best, and even if handled safely it would be likely to result in either missing or duplicated content.

In principle, I guess a print preview could be reflowed "live" in response to pref changes, but it may be more logical to treat it the same as an actual print job here.

(This is a more general version of bug 1659259.)

STR:

• In about:preferences#general / Fonts & Colors / Advanced, disable the option to "Allow pages to choose their own fonts, instead of your selections above", so that the browser will use the default fonts from Preferences.
• Open http://www.unicode.org/faq/basic_q.html.
• Do File > Print to open the tab-modal print panel with its preview; observe the preview document using the default sans-serif font.
• In another tab, go to Preferences / Fonts & Colors / Advanced, and change the default sans-serif font for Latin text to a font with very different glyph size/style -- e.g. from Arial or Helvetica to DejaVu Sans, Impact, Arial Narrow, or Verdana.
• Switch back to the Unicode-FAQ tab and observe the print preview panel.

Expected result:
Either the preview should be unchanged,
or the entire preview document should be reflowed with the new default font, potentially changing all the line breaks, page content, even total page count.

Actual result:
The text in the preview is changed to the new default font, but nothing has reflowed, so the fragments of text are poorly positioned -- with ugly gaps if the new font is more compact, or overlapping/cut off if the new font requires more space.

Comment 1

[Jonathan Kew [:jfkthame]]

Attachment: Screenshot showing broken print-preview rendering after a font prefs change

To illustrate, here's a screenshot of the preview display after changing the default sans-serif font from Helvetica to DejaVu Sans on macOS. The document is rendered using DejaVu Sans but still using the Helvetica-based layout, resulting in overlapping and clipped text.