Thanks for the report and the sample project, it made this easy to reproduce. This is a bug and it's fixed for the next Studio release.
What was happening: while a cell was being edited, Objo kept firing PaintCellText for that cell and painting your custom text on a layer above the editing field. So both were visible at once: the 13pt text drawn in the handler and the editor's own text, which uses the ListBox's font (24pt in your project) rather than the font you painted with.
The fix: while a cell is being edited, PaintCellText no longer fires for that cell and its custom drawing is suspended. The editing field is the only text shown until the edit is committed or cancelled, then your painted text returns. PaintCellBackground keeps firing normally during an edit.
One behaviour to be aware of, by design: the editing field always uses the ListBox's own FontName and FontSize, not the font your PaintCellText handler drew with. So in your project the cell edits at 24pt. If you want to edit at 13pt, set the ListBox's FontSize to 13 and paint the row that needs larger text by hand instead. This will be documented in the ListBox custom drawing section.
The next release also fixes the extra click you needed before you could type: the editing field now opens focused with its text selected, Enter commits the edit, and Escape cancels and restores the cell's previous text.
Tracked as https://feedback.objo.dev/bug/1710.
