Fix scrolling text smearing at text sizes above 1

[sebromero]

The problem

endText() scrolls by one pixel per frame and, rather than repainting the screen, erases only the single line the text just vacated. That erase has to land exactly on the trailing edge, because nothing else will cover it.

It doesn't. The travel distance is computed from textFontWidth() / textFontHeight(), which scale with textSize(), while the erase is positioned from _font->width / _font->height, which don't:

int scrollLength = _textBuffer.length() * textFontWidth() + _textX + 1;  // scaled
...
const int clearX = text_x + _textBuffer.length() * _font->width;         // unscaled
line(clearX, _textY, clearX, _textY + _font->height - 1);                // unscaled

The two agree only at textSize(1). Above that the erase falls short of the trailing edge and leaves a line of pixels behind on every frame, so glyphs smear in the scroll direction.

The fix

Use the scaled accessors for the erase as well. Six lines across the four scroll branches.

The two bitmap() calls in SCROLL_RIGHT and SCROLL_DOWN are deliberately left alone: bitmap() takes unscaled source dimensions plus separate scale factors, so passing _font->width / _font->height there is already correct.

Why horizontal scrolling looked fine

SCROLL_RIGHT was the least affected because the space-glyph bitmap() drawn just after the erase already covers the full scaled height. SCROLL_LEFT leaves a 1px-wide column behind, which is invisible whenever the trailing column of the last glyph happens to be blank. The vertical directions have no such cover, which is why they were the visible ones.

Verification

Modelling the painted and erased regions frame by frame, counting pixels from frame N that frame N+1 neither repaints nor erases (Font_5x7):

textSize	LEFT	RIGHT	UP	DOWN		LEFT	RIGHT	UP	DOWN
1 x 1	0	0	0	0	→	0	0	0	0
2 x 2	14	0	20	20	→	0	0	0	0
3 x 1	7	0	20	40	→	0	0	0	0
1 x 3	14	0	10	0	→	0	0	0	0

Zero stale pixels in every direction at every size, including the non-uniform ones from textSize(sx, sy).

Originally found on a 128x64 I2C OLED driven through a Print-based ArduinoGraphics backend: SCROLL_UP at textSize(2) tore the glyphs and left the previous word's pixels on screen. Confirmed fixed there.

🤖 Generated with Claude Code

[leonardocavagnis]

LGTM :)

Tested on 128x64 I2C OLED (Chip controller: ST7315)