Huang Xin
2.8 KiB
name, description, metadata
| name | description | metadata | ||
|---|---|---|---|---|
| pdf-text-selection-fontscale-4480 | PDF text selection/highlight misplaced (into margins, offset down) when OS font-size accessibility scaling is on |
|
Issue #4480: on some Android devices PDF text selection/highlight is misplaced — the blue selection rectangles bleed into the blank page margins and sit ~1/3 line too low. Reported on a Galaxy Tab A8; NOT reproducible on a Galaxy S21.
Root cause (NOT what it looked like): it is the OS accessibility "font size" setting (Android Settings > Display > Font size, settings put system font_scale 1.3), not the WebView version or devicePixelRatio. The OS font scale multiplies every piece of WebView-rendered text — including the transparent pdf.js text layer used for selection/highlight — but leaves the canvas page bitmap untouched. So the text-layer spans end up fontScalex larger than the glyphs baked into the canvas, and the native ::selection boxes (which follow the span boxes) overshoot the text horizontally and vertically. The Tab A8 (a tablet) had enlarged system fonts; the S21 did not.
Ruled out during investigation: WebView version (Tab A8 was on WebView 148, newer than the working S21's 147 and a WebView-124 emulator — all fine at default font scale); devicePixelRatio (the paginator's fit-width zoom keeps --total-scale-factor DPR-invariant); interactive-vs-programmatic selection (both fine). Font-metric/realm mismatch was a red herring: main-app-doc and iframe-doc measureText are identical on working devices.
Fix (packages/foliate-js/pdf.js, render()): detect the OS font scale with a probe (offsetHeight of a 100px/line-height:1 box = 100 * fontScale, unaffected by DPR or the <html> scale(1/dpr) transform) and divide it back out: --total-scale-factor = scale / getFontScale(doc). Positions (left/top) are px and font-scale-independent, so only font-size needs correcting. At font_scale 1.0 the probe returns 1.0 → no change (no regression). PDF-only; EPUB is unaffected because its text and overlay scale together.
Repro/verify harness (reusable): the release APK's WebView is CDP-debuggable. adb forward tcp:PORT localabstract:webview_devtools_remote_$(pidof com.bilingify.readest), then drive Runtime.evaluate over the page WebSocket. The PDF renders in an iframe nested inside foliate-view's shadow DOM — deep-traverse shadowRoot + iframe.contentDocument to reach .textLayer. Create a multi-line selection with doc.getSelection().addRange() + adb exec-out screencap to see the native highlight. Set settings put system font_scale 1.3 to reproduce. See android-cdp-e2e-lane.
Related PDF text-layer notes: pdf-spread-canvas-seam-4587 (the --total-scale-factor / canvas-size line this fix touches), overlayer-splitrange-textnodes.