QGIS » QGIS Application

The attached polygon layer consists of several large contour polygons with holes in them. The two largest polygons have the diagonal lines running through them -- the boundary of those polygons needs to be moved out to the clip region. Where the two diagonal lines meet at the top, 10% of the way from the left, is roughly where the two largest polygons meet.

There are several problems when viewing this polygon layer in QGIS:

(1) If the entire polygon layer is not visible in the viewport, then nothing is rendered. This means that you can't zoom in very far, because the layer disappears.

(2) In edit mode while using the Vertex Tool, the left-most large polygon (the 10% or so at the top left of the layer) cannot be selected. If you mouse over it, the red selection controls do not appear on the vertices. If you mouse over anything to the right of the leftmost 10% of the shape, then the entire shape is highlighted with red selection controls, including the leftmost 10%. Therefore there seems to be a selection bounding box around most of the layer, but not the largest polygon in the left 10%. Somehow that is excluded from selection.

(3) Sometimes without rhyme or reason, none of the polygons can be selected with the Vertex Tool, even when it is in view. Conversely, occasionally you can select vertices in the left 10%, when you are zoomed in, but it seems that you have to mouse over the polygon with the Vertex Tool to turn it red before zooming. (I don't have a reliable way to reproduce.)