iOS IME causes app/page freeze with line wrapping.

[lishid]

After receiving many bug reports of the editor freezing while typing in Chinese and Japanese, despite having no debugging capabilities once the entire app (or safari page) freezes, I have found a weird combination on https://codemirror.net/6/ that can repro this bug!

Steps to repro:

• Open https://codemirror.net/6/ on iOS (Tested with iPhone 7 and iPhone 11, iOS 14.5.2 and 14.6)
• (See picture below) Use remote debugger from macOS safari to add the css white-space: pre-wrap to the content element .cm-content.
• Zoom out to see the full width of the page. (For some reason this step seems to be required)
• Use the Chinese IME to type "a", then space to confirm the conversion, repeat until the entire browser page freezes. Sometimes this takes 20-30 tries, or until the line wraps. Sometimes this happens a few characters in.
• Notice that remote debugger is completely frozen and no commands work, including pausing script execution.
• Video attached.

Extra information that might be helpful:

• In my testing, disabling drawSelection seems to fix this issue, but it is required to use allowMultipleSelections. Disabling line wrapping also fixes it. It seems that this is a unique combination that's causing the issue.
• In the video, once it's frozen, despite the cursor still blinking, I am trying to type many keys. Later, when the keyboard is closed, I am attempting to scroll the page.
• When this issue happens, my device gets hot, suggesting that it may have ran into an infinite loop somewhere.

RPReplay_Final1622523711.mp4

[lishid]

More reports of the bug, some with videos of Chinese/Japanese:

• https://forum.obsidian.md/t/bug-ios-problem-with-writing-chinese-japanese-characters/15332/5
• https://forum.obsidian.md/t/bug-problem-with-typing-english-by-chinese-keyboard/17968/2
• https://forum.obsidian.md/t/bug-whole-ipad-ios-app-got-stuck-when-typing-english-character-with-a-chinese-input-keyboard/17536

[lishid]

A possible loop is at:

draw-selection.ts wrappedLine > cursor.ts moveToLineBoundary > blockview.ts LineView.find

[marijnh]

Well, that was fun to debug—Mobile Safari goes completely black-hole once it hits an infinite loop, not letting any information escape from its bowels. Fortunately I found a way to reliably trigger this issue, so I just kept moving early return statements around until I found out where the problem comes from. And it was a dumb bug in the library itself—see attached patch.

[lishid]

Thank you!