[canvas-] change mock window size from 80x25 to 1x1 saulpw#2171

Plotter/Canvas/GraphSheet do not have a screen when __init__() is called.
Until they have a screen, DrawablePane.windowWidth and .windowHeight
will give 25 and 80. If the actual terminal is smaller than 80x25.
errors will be caused by drawing past the edges of the terminal.

This commit uses a temporary size of 1x1 in __init__().
True canvas dimensions will be calculated later, with the real
curses screen, by first calling Canvas.refresh(), then waiting
for the next screen draw. During that draw, the sheet's screen
will be provided by drawSheet():
drawSheet(scr, sheet) -> draw(scr) -> render(h, w) -> resetCanvasDimensions(w, h)

visidata/canvas.py

@@ -145,7 +145,7 @@ def __init__(self, *names, **kwargs):
         self.labels = []  # (x, y, text, attr, row)
         self.hiddenAttrs = set()
         self.needsRefresh = False
-        self.resetCanvasDimensions(self.windowHeight, self.windowWidth)
+        self.resetCanvasDimensions(1, 1)  #2171
 
     @property
     def nRows(self):
@@ -393,6 +393,12 @@ def resetCanvasDimensions(self, windowHeight, windowWidth):
                 if self.cursorBox.xmin == self.visibleBox.xmin and self.cursorBox.ymin == self.calcBottomCursorY():
                     realign_cursor = True
         super().resetCanvasDimensions(windowHeight, windowWidth)
+        # if window is not big enough to contain a particular margin, pretend that margin is 0
+        pvbox_x = pvbox_y = 0
+        if self.plotheight > self.left_margin:
+            pvbox_x = self.left_margin
+        if self.plotheight > self.topMarginPixels:
+            pvbox_y = self.topMarginPixels
         if hasattr(self, 'legendwidth'):
             # +4 = 1 empty space after the graph + 2 characters for the legend prefixes of "1:", "2:", etc +
             #      1 character for the empty rightmost column
@@ -403,8 +409,10 @@ def resetCanvasDimensions(self, windowHeight, windowWidth):
         else:
             pvbox_xmax = self.plotwidth-self.rightMarginPixels-1
         self.left_margin = min(self.left_margin, math.ceil(self.plotwidth * 1/3)//2*2)
-        self.plotviewBox = BoundingBox(self.left_margin, self.topMarginPixels,
-                                       pvbox_xmax, self.plotheight-self.bottomMarginPixels-1)
+        # enforce a minimum plotview box size of 1x1
+        pvbox_xmax = max(pvbox_xmax, 1)
+        pvbox_ymax = max(self.plotheight-self.bottomMarginPixels-1, 1)
+        self.plotviewBox = BoundingBox(pvbox_x, pvbox_y, pvbox_xmax, pvbox_ymax)
         if [self.plotheight, self.plotwidth] != old_plotsize:
             if hasattr(self, 'cursorBox') and self.cursorBox:
                 self.setCursorSizeInPlotterPixels(2, 4)