Calibre 9.11 Viewer rendering issue after maximizing window (images become tiny or le

[mirai]

Hello,

I would like to report what appears to be a rendering issue in the Calibre Viewer.

I spent quite a long time trying to isolate the problem and created a small reproducible test EPUB. The behavior seems reproducible under specific conditions.

## Environment

- Windows 11
- Calibre 9.11
- EPUB3
- Default Viewer settings

## Attached files

- viewer_rendering_bug_test.epub
- viewer_incorrect_rendering.png

## Steps to reproduce

1. Open the attached EPUB in the Calibre Viewer.
2. The Viewer first opens in a normal (non-maximized) window.
3. Maximize the Viewer window.

## Expected result

The image should scale normally to fit the available window while preserving its aspect ratio.

## Actual result

The rendering becomes incorrect.

Two different behaviors occur depending on the page.

### Cover page

Before maximizing:
- The image almost fills the initial Viewer window.

After maximizing:
- The image becomes smaller.
- The image is aligned toward the left side of the page.

### Normal image pages (001–003)

Before maximizing:
- The image is displayed normally.

After maximizing:
- The image becomes extremely small (thumbnail-sized).
- The image remains centered.

A screenshot of this behavior is attached.

## Additional observations

The cover page and the normal image pages behave differently even though they contain exactly the same test image.

This may suggest that the Viewer uses different rendering behavior for cover pages and normal XHTML pages, although I cannot be certain.

I also observed that manual zoom operations affect the behavior.

If I use Ctrl + Mouse Wheel to zoom the image to a reasonable size, close the Viewer, and reopen the EPUB, the rendering sometimes becomes normal.

If I zoom out again until the image becomes tiny, the incorrect rendering returns.

This may suggest that an internal zoom state or rendering state is preserved and reused after maximizing the Viewer window.

## Additional investigation

I also tested:

- a minimal EPUB containing a single image,
- different image aspect ratios,
- replacing all images with identical test images,
- recreating the EPUB from scratch.

The behavior persisted in various forms.

The attached EPUB is the smallest test case I could produce while still reproducing the issue.

I should also mention that I am completely new to EPUB, Sigil, and Calibre. I am not a developer or an expert. Most of this investigation was carried out step by step with the help of ChatGPT while trying to create my own EPUB photo book. If further information is needed, I will do my best to provide it within my abilities.

Thank you very much for your work on Calibre.

[kovidgoyal]

Look at https://manual.calibre-ebook.com/con...ormatting-demo it tells you how to code full page images (among other things) in a way that will work with most epub readers.

[JSWolf]

It's not a bug and it's not poor code. It's a settings issue in the viewer. By default, when you go full screen, you get automatic columns. So your image is in the first column.

Kovid, could you have settings for columns, for when the viewer more then 1/2 the screen width and form when the view is less then 1/2 the screen width? If I want to have just 2 columns in full screen, I have to change it and when I do and I go back my windowed view, I still have the 2 columns. It would be best if full screen I had two columns but in my windowed view I had just one column. This way, I would not have to keep changing settings. Oh ans one other set of settings. An option for the space between columns. As it is, I'd have to set the margins every time I go from columns to no columns and back.. Thanks.

[DNSB]

In my calibre setup, for the viewer customization, I have the # of columns set to 1 and did not see the issues you saw.

Otherwise, epubcheck was not happy with your test ePub. I changed the naked img tags to using an SVG wrapper (my preference for cover and other full page images) and a bit of other cleanup. The revised ePub is attached to this message.

I've attached the modified ePub and some images from calibre's ebook-viewer.