[Naragod]

Proposed Changes

This fixes a blank PDF in the grading view. The page stays blank until the grader resizes the window. David asked to split #8228 in two. This PR holds the blank PDF fix.

The viewer hides itself while a file loads. pdf.js draws the pages a person can see. In a hidden pane, pdf.js finds zero pages to draw. A PDF with one page, such as a cover page, finishes while the pane is still hidden. The page then stays blank until a window resize forces a redraw.

In the code, file_viewer.jsx sets display: none while it loads. pdf.js fires pagesloaded before the pane shows. update_pdf_view then skips the refit.

• pdf_viewer.jsx now watches its pane with a ResizeObserver.
• When the pane shows, the viewer fits the pages to it.
• When the pane changes width, the viewer fits the pages again.
• After each fit, the viewer calls pdfViewer.update() to draw the pages in view.
• pdf.js skips its own redraw when the scale stays the same, so the viewer makes this call itself.
• The viewer tracks the pane's outer width, its border box.
• A scrollbar that shows up after a fit leaves that width the same, so the fit runs once.

Verification

We ran these checks in Chromium on a local scanned exam. We ran them before the split, with the #8230 fix in place too.

Check	Before	After
Runs that forced the PDF to load while hidden, then left it blank	6 of 6	0 of 6
Zoom set to 150 %, kept after a switch		Yes
Redraw loops on a window resize or a pane drag		0

Screenshots of your changes (if applicable)

Type of Change

(Write an X or a brief description next to the type or types that best describe your changes.)

Type	Applies?
🚨 Breaking change (fix or feature that would cause existing functionality to change)
✨ New feature (non-breaking change that adds functionality)
🐛 Bug fix (non-breaking change that fixes an issue)	X
🎨 User interface change (change to user interface; provide screenshots)
♻️ Refactoring (internal change to codebase, without changing functionality)
🚦 Test update (change that only adds or modifies tests)
📦 Dependency update (change that updates a dependency)
📖 Documentation update (change that updates documentation)
🔧 Internal (change that only affects developers or continuous integration)

Checklist

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Questions and Comments

• This PR and Ticket-672: Fix 404 on file download after exam switch #8230 both change pdf_viewer.jsx. Git merges them clean in either order.

[coveralls]

Coverage Report for CI Build 39974016134

Coverage increased (+0.002%) to 91.049%

Details

• Coverage increased (+0.002%) from the base build.
• Patch coverage: 9 of 9 lines across 1 file are fully covered (100%).
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	54015
Covered Lines:	50227
Line Coverage:	92.99%
Relevant Branches:	2569
Covered Branches:	1292
Branch Coverage:	50.29%
Branches in Coverage %:	Yes
Coverage Strength:	127.5 hits per line

---

💛 - Coveralls

[coveralls]

Coverage Report for CI Build 39974050132

Coverage increased (+0.002%) to 91.049%

Details

• Coverage increased (+0.002%) from the base build.
• Patch coverage: 9 of 9 lines across 1 file are fully covered (100%).
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

---

Coverage Stats

Relevant Lines:	54015
Covered Lines:	50227
Line Coverage:	92.99%
Relevant Branches:	2569
Covered Branches:	1292
Branch Coverage:	50.29%
Branches in Coverage %:	Yes
Coverage Strength:	127.5 hits per line

---

💛 - Coveralls

[david-yz-liu]

Nice work @Naragod!