Font size input field bug: Single digit becomes "5" after deleting second digit

[ZheCheng]

Describe the problem:
When modifying the font size setting in Tabby's Appearance preferences, there is an unexpected behavior when editing two-digit numbers. When deleting the second digit of a two-digit number using backspace, the remaining first digit automatically changes to "5".

To Reproduce:

1. Go to Tabby Settings -> Appearance
2. Locate the font size input field
3. Enter a two-digit number (e.g., "12")
4. Place cursor after the second digit
5. Press backspace to delete the second digit
6. Observe that the remaining first digit ("1") changes to "5"

Expected Behavior

• When deleting the second digit of a two-digit number, the first digit should remain unchanged
• In the example above, "12" should become "1" after deleting "2"

Actual Behavior

• When deleting the second digit, the first digit is automatically changed to "5"
• For example, "12" becomes "5" instead of "1" after deleting "2"

Environment

• Tabby Version: 1.0.216
• Operating System: macOS Sequoia 15.2
• Processor: Apple M1

Additional Notes

• The issue occurs specifically when using backspace to delete digits