PDF text selection on iPad difficult and visually confusing

## Bug report form
Reported by user: https://app.hubspot.com/contacts/6291320/ticket/1126006850
### Steps to reproduce
1. Using an iPad, open a PDF Hypothesis reading assignment. Example here: https://hypothesis.instructure.com/courses/258/assignments/3609
2. Touch screen to start selecting text
3. Drag the text selector over the text and note how the iPad indicates the selected text in the UI (first screenshot).
Also note that the **Annotate** icon in the adder displays as though there is no selected text, though if you tap to annotate you are brought to the sidebar to create a new annotation as usual.
Additionally, note that the _actual selected text is properly quoted_, rather than the entire page as one might mistakenly think based on the earlier visual interface (screenshot 2)

### Expected behaviour
Text selection visual UI should behave as expected without expanding to cover the entire document. Additionally, the **Annotate** icon should display as if there is selected text to annotate.

### Browser/system information
Tested on iPad (8th generation), iPadOS 15.7. Tested with both the latest version of Canvas app, and opening assignment directly in Chrome browser.